DebugInfo+DFSan: Ensure that debug info references to llvm::Functions remain pointing to the underlying function when wrappers are created

This is somewhat the inverse of how similar bugs in DAE and ArgPromo
manifested and were addressed. In those passes, individual call sites
were visited explicitly, and then the old function was deleted. This
left the debug info with a null llvm::Function* that needed to be
updated to point to the new function.

In the case of DFSan, it RAUWs the old function with the wrapper, which
includes debug info. So now the debug info refers to the wrapper, which
doesn't actually have any instructions with debug info in it, so it is
ignored entirely - resulting in a DW_TAG_subprogram with no high/low pc,
etc. Instead, fix up the debug info to refer to the original function
after the RAUW messed it up.

Reviewed/discussed with Peter Collingbourne on the llvm-dev mailing
list.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219249 91177308-0d34-0410-b5e6-96231b3b80d8
Add return value and negative checks to MapVector::erase from r219240.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219250 91177308-0d34-0410-b5e6-96231b3b80d8
[DebugInfo] Turn DWARFContext::Section into DWARFSection (NFC).

It would be more convenient to pass DWARFSection into DWARFUnitSection
constructor, instead of passing its components (Data and RelocAddrMap)
as a separate arguments.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219252 91177308-0d34-0410-b5e6-96231b3b80d8