PR37255: DebugInfo: LTO with -g inlined into -gmlt combined with Split DWARF...

PR37255: DebugInfo: LTO with -g inlined into -gmlt combined with Split DWARF without CU cross-references

A combination of features ^ that lead to a mismatch of expectations
about how a subprogram definition DIE would be produced with/without a
declaration when taking full -g debug info and inlining it into a -gmlt
CU - specifically when using Split DWARF that doesn't support cross-CU
references, so we have to put the -g debug info into the -gmlt CU, which
gets confusing about which mode is respected.

This patch comes down on respecting the CU the debug info is emitted
into, rather than preserving the full debug info when it's emitted into
the gmlt CU.