[ffigen] C++ functions outside extern "C" are bound to their unmangled name #3666

When parsing in C++ mode, ffigen binds a free function declared outside any
extern "C" block to its plain spelling, but the exported symbol is mangled.
Func uses originalName as the lookup symbol (func.dart), and
originalName is the cursor spelling, so C++ linkage is never accounted for.

Example from the existing test suite: int outside(double d) in
test/native_cpp_test/cpp_extern_c_test.h generates a lookup of 'outside',
while the real symbol is ?outside@@YAHN@Z (MSVC) / _Z7outsided (GCC/Clang).
Calling it at runtime fails with a symbol-not-found error. The test only checks
isA<int Function(double)>() and never calls the function, so this goes
unnoticed.

The same applies to Global (global.dart) for variables declared outside
extern "C".

Possible fixes, both with trade-offs:

• Emit an extern "C" shim via the existing toCppBindingString mechanism
  used by CppClass methods. ABI-portable, but requires compiling the
  generated .cpp, and globals become accessor functions.
• Bind clang_Cursor_getMangling and look up the mangled symbol. Simpler, but
  the result is ABI-specific to the host that generated the bindings.

At minimum, the test should call outside so the failure is visible, or
ffigen should warn when a C++-linkage function is bound by its unmangled name.