DebugInfo: Accurately propagate the section used by a relocation when...

DebugInfo: Accurately propagate the section used by a relocation when accessing ranges defined by low/high_pc

This is difficult/not possible to test in LLVM, but is visible as a
crash in LLD when parsing DWARF to generate gdb-index.

This function is called by llvm-dwarfdump when parsing high_pc for
non-verbose output (to print the actual high_pc rather than the low_pc
relative value), but in that case llvm-dwarfdump doesn't print section
names (if it did, it would hit this problem).

We could add some other features to llvm-dwarfdump to expose this, but
nothing really springs to my mind. I will add a test to lld, though.

llvm-svn: 350010