1. With a Clang that doesn't default to GNU extensions they need to be enabled explicitly.
2. The X86 directory lit config sets it already, there's no reason for this test to do it by itself.
3. The C frontend executable will fail if there's for example a Clang resource file for the C++ mode that sets C++-specific options:
```
+ /home/tambre/dev/llvm/build/bin/clang --target=x86_64-unknown-linux-gnu -fPIE -fuse-ld=lld -Wl,--unresolved-symbols=ignore-all -pie -fPIC -shared /home/tambre/dev/llvm/bolt/test/R_ABS.pic.lld.cpp -o /home/tambre/dev/llvm/build/tools/bolt/test/Output/R_ABS.pic.lld.cpp.tmp.so -Wl,-q -fuse-ld=lld
clang: warning: argument unused during compilation: '-pie' [-Wunused-command-line-argument]
error: invalid argument '-std=c23' not allowed with 'C++'
```
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