Only resolve a macro's instantiation loc once per macro, instead of once

per token lexed from it.  This speeds up clang on c99-intconst-1.c from
the GCC testsuite from 3.64s to 2.66s (36%).  This reduces the number of
binary search FileID lookups from 251570522 to 114175649 on this testcase.

llvm-svn: 62992