[clangd] Exclude builtin headers from system include extraction

Most headers that we discover are likely to be fairly portable across at least
clang/gcc, which is why we get away with adding them to clangd's search path.

This is not the case for the compiler builtin headers, which are shipped with
the parser and rely on parser builtins/features. We're better off *not* using
the the scanned builtin headers, and hoping our own intrinsics provide the
interface the code is relying on.

Fixes https://github.com/clangd/clangd/issues/1695

Differential Revision: https://reviews.llvm.org/D156044