Eagerly resolve the spelling location of the tokens in a definition

of a macro.  Since these tokens may themselves be from macro 
expansions, we need to resolve down to the spelling loc when the
macro ends up being instantiated.  Instead of resolving this for
each token expanded from the macro definition, just do it once when
the macro is defined.  This speeds up clang on c99-intconst-1.c from
2.66s to 2.43s (9.5%), reducing the FileID lookups from 407244 linear and
114175649 binary to 2529040 linear and 64771121 binary.

llvm-svn: 62993