suppress invalid-unevaluated-string for AppleClang 16

[wkgcass]

Description

On x64 macOS 15.1.1 and Xcode 16.2, I have Apple Clang 16.0.0 insalled:

# clang --version
Apple clang version 16.0.0 (clang-1600.0.26.6)
Target: x86_64-apple-darwin24.1.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

When compiling msquic, I can see errors produced by -Winvalid-unevaluated-string, just like the following one:

msquic/src/core/quicdef.h:243:66: error: encoding prefix 'L' on an unevaluated string literal has no effect [-Werror,-Winvalid-unevaluated-string]
  243 | CXPLAT_STATIC_ASSERT(IS_POWER_OF_TWO(QUIC_MAX_RANGE_ALLOC_SIZE), L"Must be power of two");
      |                                                                  ^

The xcode 16 release note says that:

The following C++26 features have been implemented:
...
Unevaluated strings (P2361R6)

It seems that from this version the invalid-unevaluated-string warning began to appear.

Testing

I can compile without any error with this patch.

No further tests required I think.

Documentation

None.