[Preamble] Invalidate preamble when missing headers become present.

Summary:
To avoid excessive extra stat()s, only check the possible locations of
headers that weren't found at all (leading to a compile error).
For headers that *were* found, we don't check for files earlier on the
search path that could override them.

Reviewers: kadircet

Subscribers: javed.absar, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77942