Refactor the paths used for checking and getting the spelling of #include

filenames (and also '#pragma GCC dependency' of course).  Now, assuming
no cleaning is needed, we can go all the way from lexing the filename to
doing filename lookups with no mallocs.  This speeds up user PP time from
0.077 to 0.075s for Cocoa.h (2.6%).

llvm-svn: 39092