Clean up whitespace checks

In TokenLexer::ExpandFunctionArguments(), CurTok.hasLeadingSpace() is
checked in multiple locations, each time subtly differently. Checking it
early, when the token is seen, and using NextTokGetsSpace exclusively
after that makes the code simpler.

No change in behaviour is intended.

Patch by Harald van Dijk!

llvm-svn: 200788