[PATCH] lex: improve include handling on Linux for Windows

[Richard Smith]

  This patch seems to be missing tests.

  Also, a 1K stack buffer seems really excessive. `#include`d paths rarely need more than 80 columns; 128 bytes would seem ample. (The other two `Path` variables are for include paths, I think, so might be longer (but even then, they also seem excessive).

http://llvm-reviews.chandlerc.com/D2994