Suppress elided variadic macro argument extension diagnostic for macros using

the related comma pasting extension.

In certain cases, we used to get two diagnostics for what is essentially one
extension.  This change suppresses the first diagnostic in certain cases
where we know we're going to print the second diagnostic.  The
diagnostic is redundant, and it can't be suppressed in the definition
of the macro because it points at the use of the macro, so we want to
avoid printing it if possible.

The implementation works by detecting constructs which look like comma
pasting at the time of the definition of the macro; this information
is then used when the macro is used.  (We can't actually detect
whether we're using the comma pasting extension until the macro is
actually used, but we can detecting constructs which will be comma
pasting if the varargs argument is elided.)

<rdar://problem/12292192>

llvm-svn: 167907