Adjust doc and diag

Hello, I am back now and will be available anytime next week, so if anyone has any idea on this topic, please at me!

[clang] add -Wvla-stack-allocation and divide different VLA warning

int foo(void);

void bar(int a, int b[*]); // variable length array used, correct
void bar(int a, int b[a]) { variable length array used, correct
  int x[foo()]; // variable length array that may require stack allocation used, correct

  (void)sizeof(int[foo()]); // variable length array that may require stack allocation used, incorrect and the diagnostic triggers twice

  int (*y)[foo()]; // variable length array that may require stack allocation used, incorrect, this is a pointer to a VLA
}

Improved comments and release notes

Update release notes.

Change implements according reviews.

In D137043#3935129, @aaronpuchert wrote:

This include-if-exists mechanism seems brittle to me.

ping :)

ping :)

After digging more deeply into tests, I found that if we decide to make clang include stdc-predef.h or other things, the behavior of many libtooling-related tools (e.g. clang-tidy, clangd, and python binding) will be affected.

I'm sorry that my patch can not pass the CI test, but I can not reproduce the CI
failure loaclly, including clang-format related problems. On my machine,
ninja check-clang shows: