Fix build issues with clang18

[Raphtaliyah]

Clang 18 introduced some changes that prevent building CoreCLR:

• The warning for VLAs being a C++ extension is now on by default which causes errors with -Werror. Enabled -Wvla-extension by default in C++ compilation modes llvm/llvm-project#62836
• stdarg.h now has a guard that depends on va_arg not being defined, currently the PAL internal doesn't undefined that macro (despite PAL defining it) which prevent stdarg.h from being included again and causes missing symbols (va_start, va_end, va_arg).

This PR fixes the warning by disabling it to match the behaviour of previous clang versions and undefines va_arg in palinternal.h.

[Raphtaliyah]

@dotnet-policy-service agree

[jkotas]

Where do we use VLAs?

[Raphtaliyah]

There are two instance when building for Linux:

runtime/src/coreclr/gc/unix/gcenv.unix.cpp

Line 635 in 590d664

unsigned long nodeMask[nodeMaskLength];

and

runtime/src/coreclr/pal/src/file/path.cpp

Line 402 in 590d664

char TempBuffer[nBufferLength > 0 ? nBufferLength : 1];

[jkotas]

If it is just these two, I think it is better to convert them to use alloca.

[Raphtaliyah]

That should be easy, I'll change them tomorrow! Should I also check if there are more in code that's only used for different platforms? Also if there are no notes in the coding guidelines it might be worth mentioning that VLAs shouldn't be used (and maybe enable warnings for them?)

[jkotas]

That should be easy, I'll change them tomorrow!

Thanks!

Should I also check if there are more in code that's only used for different platforms?

I do not expect that you will find any. VLAs are not supported on Windows that prohibits their use in majority of the code. And there is very little code in the repo that is non-Windows non-Linux.

there are no notes in the coding guidelines it might be worth mentioning that VLAs shouldn't be used (and maybe enable warnings for them

VLAs are very niche. I do not think it is worth mentioning them in the coding guidelines. Enabling the warnings for them on older compilers sounds good if it is possible.

[Raphtaliyah]

Converted them to alloca and enabled warnings for unix hosts. If I understand it correctly only MSVC is supported for Windows which doesn't support VLAs (for C++), for Linux there is Clang and GCC, both of which support -Wvla, this should cover every supported compiler/platform.

[jkotas]

Thank you!