Reland "[clang][modules] Move `UNHASHED_CONTROL_BLOCK` up in the AST file"

This reverts commit b6ba804f, effectively relanding commit 7d156572.

The original commit incorrectly called `ASTWriter::writeUnhashedControlBlock()` before `ASTWriter::collectNonAffectingInputFiles()`, causing SourceLocations/FileIDs in the pragma diagnostic mappings block to be invalid. This is now tested by `clang/test/Modules/diag-mappings-affecting.c`.