revert r133003 and fix the bug properly: the issue was that ## in a token

lexer is not a paste operator, it is a normal token.  This fixes a conformance
issue shown here:
http://p99.gforge.inria.fr/c99-conformance/c99-conformance-clang-2.9.html

and it defines away the crash from before.

llvm-svn: 133005