[llvm] [clang] [libcxx] [mlir] [compiler-rt] [sanitizer] Skip /include/c++/ from summary (PR #78534)

[Sam James via cfe-commits]

thesamesam wrote:

Note that on Gentoo, this isn't right either, we have e.g.:
```
/usr/lib/gcc/x86_64-pc-linux-gnu/14/include/g++-v14/bits/new_allocator.h
/usr/lib/gcc/x86_64-pc-linux-gnu/14/include/g++-v14/ext/new_allocator.h
```

https://github.com/llvm/llvm-project/pull/78534