[modules] Fix error "malformed or corrupted AST file: 'SourceLocation remap...

[modules] Fix error "malformed or corrupted AST file: 'SourceLocation remap refers to unknown module...'".

When a framework can be found at a new location, all references to it in
the module cache become outdated. When we try to load such outdated .pcm
file, we shouldn't change any already loaded and processed modules.

If `Module` has `ASTFile`, it means we've read its AST block already and
it is too late to undo that. If `ASTFile` is `None`, there is no value
in setting it to `None` again. So we don't reset `ASTFile` in
`ModuleManager::removeModules` at all.

rdar://97216258

Differential Revision: https://reviews.llvm.org/D134249