RFA: Fix pch/system-1.c failures for systems with non-trivial BUILD_VA_LIST_TYPE

[Joern Rennecke]

Neil Booth wrote:
> Comparing strings to <built-in> is always a bug.  Is your whole patch
> really the right fix?

Setting input_location.file to NULL has the effect that declarations
are recorded as coming from the input file "<built-in>".  That makes
sense for the declarations that are generated globally by the frontend
to share by compilation of all or none input files.

And it seems wrong to put the current working directory into the list
of directories to search for declarations from "<built-in>" .
Do you have another test to propose to avoid this?
	
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