gdb: change subfile::name and buildsym_compunit::m_comp_dir to strings

gdb: change subfile::name and buildsym_compunit::m_comp_dir to strings

Change subfile::name to be a string, for easier memory management.
Change buildsym_compunit::m_comp_dir as well, since we move one in to
the other at some point in patch_subfile_names, so it's easier to do
both at the same time.  There are various NULL checks for both fields
currently, replace them with empty checks, I think it ends up
equivalent.

I can't test the change in xcoffread.c, it's best-effort.

Change-Id: I62b5fb08b2089e096768a090627ac7617e90a016