eagerly resolve the spelling locations of macro argument preexpansions.

This reduces fsyntax-only time on c99-intconst-1.c from 2.43s down to 
2.01s (20%), reducing the number of fileid lookups from 2529040 linear 
and 64771121 binary to 5625902 linear and 4151182 binary.

This knocks getFileID down to only 4.6% of compile time on this testcase.
At this point, malloc/free is over 35% of compile time, primarily allocating
MacroArgs objects and their argument preexpansion vectors.

I don't feel like malloc avoiding right now, so I'm just going to call
this good.

llvm-svn: 62994