gdb: remove symtab::dirname

gdb: remove symtab::dirname

I think the symtab::dirname method is bogus, or at least very
misleading.  It makes you think that it returns the directory that was
used to find that symtab's file during compilation (i.e. the directory
the file refers to in the DWARF line header file table), or the
directory part of the symtab's filename maybe.  In fact, it returns the
compilation unit's directory, which is the CWD of the compiler, at
compilation time.  At least for DWARF, if the symtab's filename is
relative, it will be relative to that directory.  But if the symtab's
filename is absolute, then the directory returned by symtab::dirname has
nothing to do with the symtab's filename.

Remove symtab::dirname to avoid this confusion, change all users to
fetch the same information through the compunit.  At least, it will be
clear that this is a compunit property, not a symtab property.

Change-Id: I2894c3bf3789d7359a676db3c58be2c10763f5f0