[Mem2Reg] Create merged debug locations for inserted phis

Track the debug locations of the incoming values to newly-created phis,
and apply merged debug locations to the phis.

A merged location will be on line 0, but will have the correct scope
set. This improves crash reporting when an inlined instruction with a
merged location triggers a machine exception. A debugger will be able to
narrow down the crash to the correct inlined scope, instead of simply
pointing to the outer scope of the caller.

Taken together with a change allows generating merged line-0 locations
for  instructions which aren't calls, this results in a 0.5% increase in
the uncompressed size of the .debug_line section of a stage2+Release
build of clang (-O3 -g).

rdar://33858697

Differential Revision: https://reviews.llvm.org/D45397

llvm-svn: 330227