[SantizerCoverage] handle missing DBG MD when inserting libcalls

Instruction::getDebugLoc can return an invalid DebugLoc. For such cases
where metadata was accidentally removed from the libcall insertion
point, simply insert a DILocation with line 0 scoped to the caller. When
we can inline the libcall, such as during LTO, then we won't fail a
Verifier check that all calls to functions with debug metadata
themselves must have debug metadata.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D100158