Claiming a mismatch is always in a precompiled header is wrong and
misleading as a mismatch can happen in any provided AST file. Emitting a
path for a file with a problem allows to disambiguate between multiple
input files.
Use generic term "AST file" because we don't always know a kind of the
provided file (for example, see `ASTReader::readASTFileControlBlock`).
rdar://65005546
It's useful to be able to load explicitly-built PCH files into an implicit build (e.g. during dependency scanning). That's currently impossible, since the explicitly-built PCH has an empty modules cache path, while the current compilation has (and needs to have) a valid path, triggering an error in the `PCHValidator`.
This patch adds a preprocessor option and command-line flag that can be used to omit this check.
Reviewed By: dexonsmith
Differential Revision: https://reviews.llvm.org/D103802
