De-unionize fields in Token class.

 - This is fairly gross but although the code is conceptually the
   same, introducting the union causes gcc 4.2 on x86 (darwin, if that
   matters) to pessimize LexTokenInternal which is critical to our
   preprocessor performance.

This speeds up -Eonly lexing of Cocoa.h by ~4.7% in my timings and
reduces the code size of LexTokenInternal by 8.6%.

llvm-svn: 59725