[DebugInfo] Fully implement DWARF issue 180201.1 (#149226)HEADmain

Finish making LLVM's implementation of `DW_LNCT_LLVM_source` conform to
the final accepted version of `DW_LNCT_source` from
https://dwarfstd.org/issues/180201.1.html

This is effectively a continuation of a few commits which have moved in
this direction already, including:

* c9cb4fc761cd7 [DebugInfo] Store optional DIFile::Source as pointer
* 87e22bdd2bd6d Allow for mixing source/no-source DIFiles in one CU

This patch:

* Teaches LLParser that there is a distinction between an empty and an
absent "source:" field on DIFile.
* Makes printing the "source:" field in AsmWriter conditional on it
being present, instead of being conditional on it being non-empty.
* Teaches MC to map an empty-but-present source field to "\n" (which is
ambiguous, making the source strings "" and "\n" indistinguishable, but
that's what the DWARF issue specifies).

Add a test for round-tripping an empty source field through
assembler/bitcode.

Extend the test for the actual DWARF generation so it covers all of the
cases (absent, present-but-empty,
present-and-ambiguously-single-newline, present).

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