This addresses an issue with how the PCH preable works, specifically:
1. When using a PCH/preamble the module hash changes and a different cache directory is used
2. When the preamble is used, PCH & PCM validation is disabled.
Due to combination of #1 and #2, reparsing with preamble enabled can end up loading a stale module file before a header change and using it without updating it because validation is disabled and it doesn’t check that the header has changed and the module file is out-of-date.
rdar://72611253
Differential Revision: https://reviews.llvm.org/D95159
Argyrios Kyrtzidis