[release/9.0-staging] Add flags when the clang's major version is > 20.0 (#121151)

## Customer Impact

- [x] Customer reported
- [ ] Found internally

These issues were reported in
#119706 as problems with
clang-21 on Fedora 43. The investigation uncovered that clang introduced
a potentially breaking change in clang-20 that we do not currently
consume. These build changes impact VMR related builds when linux
distrobutions performing source build adopt clang-21.

clang-20 breaking change log -
https://releases.llvm.org/20.1.0/tools/clang/docs/ReleaseNotes.html#potentially-breaking-changes.

This PR contains the minimal changes needed to fix issues from the
following PR #120775.

.NET 10: #121124
.NET 8: #121150

## Regression

- [ ] Yes
- [x] No

Build with the new clang-21 compiler will cause the runtime to crash.

## Testing

This has been validated using various legs and examples to demonstrate
the usage of undefined behavior these flags convert into "defined"
behavior in C/C++.

## Risk

Low. This has zero impact on our production build since we specifically
target clang-18. This is only valid for those partners that are using
clang-20+.

Author: AaronRobinsonMSFT

eng/native/configurecompiler.cmake

@@ -526,9 +526,21 @@ if (CLR_CMAKE_HOST_UNIX)
   # Disable frame pointer optimizations so profilers can get better call stacks
   add_compile_options(-fno-omit-frame-pointer)
 
-  # Make signed arithmetic overflow of addition, subtraction, and multiplication wrap around
-  # using twos-complement representation (this is normally undefined according to the C++ spec).
-  add_compile_options(-fwrapv)
+  if((CMAKE_C_COMPILER_ID STREQUAL "Clang" AND CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 20.0) OR
+      (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 20.0))
+    # Make signed overflow well-defined. Implies the following flags in clang-20 and above.
+    # -fwrapv - Make signed arithmetic overflow of addition, subtraction, and multiplication wrap around
+    # using twos-complement representation (this is normally undefined according to the C++ spec).
+    # -fwrapv-pointer - The same as -fwrapv but for pointers.
+    add_compile_options(-fno-strict-overflow)
+
+    # Suppress C++ strict aliasing rules. This matches our use of MSVC.
+    add_compile_options(-fno-strict-aliasing)
+  else()
+    # Make signed arithmetic overflow of addition, subtraction, and multiplication wrap around
+    # using twos-complement representation (this is normally undefined according to the C++ spec).
+    add_compile_options(-fwrapv)
+  endif()
 
   if(CLR_CMAKE_HOST_APPLE)
     # Clang will by default emit objc_msgSend stubs in Xcode 14, which ld from earlier Xcodes doesn't understand.

src/coreclr/debug/di/rspriv.h

@@ -6404,8 +6404,8 @@ class CordbThread : public CordbBase, public ICorDebugThread,
     // Lazily initialized.
     EXCEPTION_RECORD *  m_pExceptionRecord;
 
-    static const CorDebugUserState kInvalidUserState = CorDebugUserState(-1);
-    CorDebugUserState     m_userState;  // This is the current state of the
+    static const int kInvalidUserState = -1;
+    int                   m_userState;  // This is the current state of the
                                         // thread, at the time that the
                                         // left side synchronized
 

src/coreclr/debug/di/rsthread.cpp

@@ -783,7 +783,7 @@ CorDebugUserState CordbThread::GetUserState()
         m_userState = pDAC->GetUserState(m_vmThreadToken);
     }
 
-    return m_userState;
+    return (CorDebugUserState)m_userState;
 }
 
 
@@ -887,7 +887,7 @@ HRESULT CordbThread::CreateStepper(ICorDebugStepper ** ppStepper)
 //Returns true if current user state of a thread is USER_WAIT_SLEEP_JOIN
 bool CordbThread::IsThreadWaitingOrSleeping()
 {
-    CorDebugUserState userState = m_userState;
+    int userState = m_userState;
     if (userState == kInvalidUserState)
     {
         //If m_userState is not ready, we'll read from DAC only part of it which
@@ -3721,14 +3721,14 @@ HRESULT CordbUnmanagedThread::SetupFirstChanceHijackForSync()
     LOG((LF_CORDB, LL_INFO10000, "CUT::SFCHFS: hijackCtx started as:\n"));
     LogContext(GetHijackCtx());
 
-    // Save the thread's full context for all platforms except for x86 because we need the 
+    // Save the thread's full context for all platforms except for x86 because we need the
     // DT_CONTEXT_EXTENDED_REGISTERS to avoid getting incomplete information and corrupt the thread context
     DT_CONTEXT context;
-#ifdef TARGET_X86    
+#ifdef TARGET_X86
     context.ContextFlags = DT_CONTEXT_FULL | DT_CONTEXT_EXTENDED_REGISTERS;
 #else
     context.ContextFlags = DT_CONTEXT_FULL;
-#endif 
+#endif
 
     BOOL succ = DbiGetThreadContext(m_handle, &context);
     _ASSERTE(succ);
@@ -3739,7 +3739,7 @@ HRESULT CordbUnmanagedThread::SetupFirstChanceHijackForSync()
         LOG((LF_CORDB, LL_ERROR, "CUT::SFCHFS: DbiGetThreadContext error=0x%x\n", error));
     }
 
-#ifdef TARGET_X86    
+#ifdef TARGET_X86
     GetHijackCtx()->ContextFlags = DT_CONTEXT_FULL | DT_CONTEXT_EXTENDED_REGISTERS;
 #else
     GetHijackCtx()->ContextFlags = DT_CONTEXT_FULL;