Reapply "llvm-symbolizer: Fix "start file" to work with Split DWARF"

Originally committed as 04c203e3
Reverted in 76851063 due to the test
failing when encountering windows directory separators.

Fix the path separator platform issue with a FileCheck pattern {{[/\\]}}

Original commit message:

A followup to the feature added in 69da27c7
that added the optional "start file name" to match "start line" - but this
didn't work with Split DWARF because of the need for the decl file number
resolution code to refer back to the skeleton unit to find its .debug_line
contribution. So this patch adds the necessary infrastructure to track the
skeleton unit corresponding to a split full unit for the purpose of this
lookup.