gdb: make symtab::compunit return a reference

The compunit symtab backlink of a symtab is never null (the constructor
asserts it), so make symtab::compunit return a reference instead of a
pointer, and have the symtab constructor take the compunit_symtab as a
reference too.  Update all callers accordingly.

This came up earlier in review, where a caller would check the result of
`symtab->compunit ()` for nullptr, and I pointed out that it was
unnecessary.  Returning a reference makes this clear.

Change-Id: Idf3a6b5fb07a10cd161826ae8a7b826d95bd96c6
Reviewed-By: Tom de Vries <tdevries@suse.de>