Fix whitespace handling in ## operator

In x ## y, where x and y are regular tokens, whitespace between x and ##
is ignored, and whitespace between ## and y is also ignored. When either
x or y is a function argument, whitespace was preserved, but it should
not be. This patch removes the checks for whitespace before ## and
before y, and in the special case where x is an empty macro argument and
y is a regular token, actively removes whitespace before y.

One existing test is affected by that change, but as clang's output now
matches the standard's requirements and that of GCC, I've tweaked the
testcase.

Patch by Harald van Dijk!
Fix whitespace handling in empty macro arguments

When a function-like macro definition ends with one of the macro's
parameters, and the argument is empty, any whitespace before the
parameter name in the macro definition needs to be preserved. Promoting
the existing NextTokGetsSpace to a preserved bit-field and checking it
at the end of the macro expansion allows it to be moved to the first
token following the macro expansion result.

Patch by Harald van Dijk!
Fix whitespace handling in empty macro expansions

When a macro expansion does not result in any tokens, and the macro name
is preceded by whitespace, the whitespace should be passed to the first
token that follows the macro expansion. Similarly when a macro expansion
ends with a placemarker token, and that placemarker token is preceded by
whitespace. This worked already for top-level macro expansions, but is
now extended to also work for nested macro expansions.

Patch by Harald van Dijk!
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!