Besides saving some calls to stat(), this also causes problems when
the user specifies a search path that Clang already adds by default,
like $SDKROOT/Library/Frameworks/. Why? Because Swift adds its search
paths after Clang has already configured its defaults, but Clang
reconfigures its search paths from scratch when compiling a module to
a PCM file to cache. This led to system search paths being found
sooner in the primary Clang instance than in the PCM files, which in
turn resulted in the PCM files being considered out of date.
This isn't likely to affect people much in practice, but it's better
to get right. (We ran into this during Doug's experiments in making
/System/Library/PrivateFrameworks a default search path in Clang
r313317; turns out that's problematic for other reasons as well.)
rdar://problem/34664596
