[ATfE] Backport llvm-libc build stability fixes from main (arm#450)

This adds the following three commits from llvm-project main to the
patch collection used for ATfE. The effect of the commits is to fix
missing dependencies in the LLVM libc build scripts, preventing an
intermittent build failure when a compile step requiring stdint.h (or
occasionally another header) is run concurrently with the generation
step that _writes_ stdint.h.

With these changes, `ninja -t missingdeps` in the libc build tree now
reports no problems.


llvm/llvm-project@66603dd
llvm/llvm-project@42017c6
llvm/llvm-project@eb04b69

Author: statham-arm