When using explicit Clang modules, some declarations might unexpectedly become invisible.
This is caused by the mechanism that loads PCM files passed via `-fmodule-file=<path>` and creates an `IdentifierInfo` for the module name. The `IdentifierInfo` creation takes place when the `ASTReader` is in a weird state, with modules that are loaded but not yet set up properly. This patch delays the creation of `IdentifierInfo` until the `ASTReader` is done with reading the PCM.
Note that the `-fmodule-file=<name>=<path>` form of the argument doesn't suffer from this issue, since it doesn't create `IdentifierInfo` for the module name.
Reviewed By: dexonsmith
Differential Revision: https://reviews.llvm.org/D111543
Jan Svoboda