Change objfile::partial_symtabs to be a unique_ptr

A plan I had a while ago was to write the DWARF index in a worker
thread.  This is why objfile::partial_symtabs is a shared_ptr.

However, it turned out that doing this required keeping the objfile
alive as well.  Now that objfiles are managed using shared_ptr,
there's no need for partial_symtabs to be one as well, so this patch
reverts that change.

gdb/ChangeLog
2019-12-12  Tom Tromey  <tom@tromey.com>

	* objfiles.h (struct objfile) <partial_symtabs>: Now a
	unique_ptr.

Change-Id: I3d7831006c40d4c8f3173ba51c0c1b0a32021ae5