[SanitizerCoverage] Add fallback DebugLocation for instrumented calls

When building the kernel with LTO, KCOV & debug information enabled,
multiple inlinable SanitizerCoverage functions require debug information
present.

In such cases we repurpose the InstrumentationIRBuilder that ensures
the necessary debug information is added if necessary.

This has been done analogous to the work for the ThreadSanitizer
in D124937.

Bug: https://github.com/ClangBuiltLinux/linux/issues/1721

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D155377