Have expand_symtabs_matching work for already-expanded CUs

Have expand_symtabs_matching work for already-expanded CUs

Currently, gdb will search the already-expanded symtabs in one loop,
and then also expand matching symtabs in another loop.  However, this
is somewhat inefficient -- when searching the already-expanded
symtabs, all such symtabs are examined.  However, the various "quick"
implementations already know which subset of symtabs might have a
match.

This changes the contract of expand_symtabs_matching to also call the
callback for an already-expanded symtab.  With this change, and some
subsequent enabling changes, the number of searched symtabs should
sometimes be reduced.  This also cuts down on the amount of redundant
code.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16994
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16998
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30736
Acked-By: Simon Marchi <simon.marchi@efficios.com>