Suppress warnings if their instantiation location is in a system header, not

their spelling location.  This prevents warnings from being swallowed just
because the caret is on the first parenthesis in, say, NULL.

This is an experiment;  the risk is that there might be a substantial number
of system headers which #define symbols to expressions which inherently cause
warnings.  My theory is that that's rare enough that it can be worked
around case-by-case, and that producing useful warnings around NULL is worth 
it.  But I'm willing to accept that I might be empirically wrong.

llvm-svn: 95870