Keep track of which source locations are part of a macro argument

instantiation and improve diagnostics which are stem from macro
arguments to trace the argument itself back through the layers of macro
expansion.

This requires some tricky handling of the source locations, as the
argument appears to be expanded in the opposite direction from the
surrounding macro. This patch provides helper routines that encapsulate
the logic and explain the reasoning behind how we step through macros
during diagnostic printing.

This fixes the rest of the test cases originially in PR9279, and later
split out into PR10214 and PR10215.

There is still some more work we can do here to improve the macro
backtrace, but those will follow as separate patches.

llvm-svn: 134660

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