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b768a8c1db
Since b4c83a13f664582015ea22924b9a0c6290d41f5b, `Preprocessor` and `Lexer` are aware of the concept of scanning dependency directives. This makes it possible to scan for them on-demand rather than eagerly on the first filesystem operation (open, or even just stat). This might improve performance, but is also necessary for the "PCH as module" mode. Some precompiled header sources use the ".pch" file extension, which means they were not getting scanned for dependency directives. This was okay when the PCH was the main input file in a separate scan step, because there we just lex the file in a scanning-specific frontend action. But when such source gets treated as a module implicitly loaded from a TU, it will get compiled as any other module - with Sema - which will result in compilation errors. (See attached test case.) rdar://107663951