Improve the time it takes to generating dwarf for assembly source files
that have been run through the 'C' pre-processor.

The implementation of SrcMgr.FindLineNumber() is slow but OK if
it uses its cache when called multiple times with an SMLoc that is
forward of the previous call.

In the case of generating dwarf for assembly source files that have
been run through the 'C' pre-processor we need to calculate the
logical line number based on the last parsed cpp hash file line
comment.  And the current code calls SrcMgr.FindLineNumber()
twice to do this causing its cache not to work and results in very
slow compile times:

% time /Volumes/SandBox/build-llvm/Debug+Asserts/bin/llvm-mc -triple thumbv7-apple-ios -filetype=obj -o /tmp/x.o mscorlib.dll.E -g
672.542u 0.299s 11:13.15 99.9%	0+0k 0+2io 2106pf+0w

So we save the info from the last parsed cpp hash file line comment
to avoid making the second call to SrcMgr.FindLineNumber() most times
and end up with compile times like:

% time /Volumes/SandBox/build-llvm/Debug+Asserts/bin/llvm-mc -triple thumbv7-apple-ios -filetype=obj -o /tmp/x.o mscorlib.dll.E -g
3.404u 0.104s 0:03.80 92.1%	0+0k 0+3io 2105pf+0w

rdar://14156934