[Coverage] Fix crash when handling certain macro expansions

When handling 'if' statements, we crash if the condition and the consequent
branch are spanned by a single macro expansion.

The crash occurs because of a sanity 'reset' in popRegions(): if an expansion
exactly spans an entire region, we set MostRecentLocation to the start of the
expansion (its 'include location'). This ensures we don't handleFileExit()
ourselves out of the expansion before we're done processing all of the regions
within it. This is tested in test/CoverageMapping/macro-expressions.c.

This causes a problem when an expansion spans both the condition and the
consequent branch of an 'if' statement. MostRecentLocation is updated to the
start of the 'if' statement in popRegions(), so the file for the expansion
isn't exited by the time we're done handling the statement. We then crash with
'fatal: File exit not handled before popRegions'.

The fix for this is to detect these kinds of expansions, and conservatively
update MostRecentLocation to the end of expansion region containing the
conditional. I've added tests to make sure we don't have the same problem with
other kinds of statements.

rdar://problem/23630316

Differential Revision: http://reviews.llvm.org/D16934

llvm-svn: 260129