1) refactor some code.

2) Add support for lexing imaginary constants (a GCC extension):

t.c:5:10: warning: imaginary constants are an extension
  A = 1.0iF;
         ^

3) Make the 'invalid suffix' diagnostic pointer more accurate:

t.c:6:10: error: invalid suffix 'qF' on floating constant
  A = 1.0qF;
         ^

instead of:

t.c:6:10: error: invalid suffix 'qF' on floating constant
  A = 1.0qF;
      ^

llvm-svn: 41411