[Preamble] Check system dependencies in preamble too
Summary:
PrecompiledPreamble hasn't checked if the system dependencies changed
before. This resulted in invalid preamble not being rebuilt if headers
that changed were found in -isystem include paths.
This pattern is sometimes used to avoid showing warnings in third
party code, so we want to correctly handle those cases.
Tested in clangd, see the follow-up patch.
Reviewers: sammccall, ioeric
Reviewed By: sammccall
Subscribers: omtcyfz, cfe-commits
Differential Revision: https://reviews.llvm.org/D48946
llvm-svn: 336528