Handle link of NoDebug CU with a CU that has debug emission enabled

Summary:
This is an issue both with regular and Thin LTO. When we link together
a DICompileUnit that is marked NoDebug (e.g when compiling with -g0
but applying an AutoFDO profile, which requires location tracking
in the compiler) and a DICompileUnit with debug emission enabled,
we can have failures during dwarf debug generation. Specifically,
when we have inlined from the NoDebug compile unit into the debug
compile unit, we can fail during construction of the abstract and
inlined scope DIEs. This is because the SPMap does not include NoDebug
CUs (they are skipped in the debug_compile_units_iterator).

This patch fixes the failures by skipping locations from NoDebug CUs
when extracting lexical scopes.

Reviewers: dblaikie, aprantl

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D29765

llvm-svn: 295384