Allow for mixing source/no-source DIFiles in one CU

The DWARF proposal that the DW_LNCT_LLVM_source extension is based on
(https://dwarfstd.org/issues/180201.1.html) allows to mix source and
non-source files in the same CU by storing an empty string as a
sentinel value.

This patch implements this feature.

Review in https://github.com/llvm/llvm-project/pull/73877