Sync dlang demangling tests from upstream libiberty testsuite.

Rationale behind the change instead of adding a `.init$' postfix being
that "initializer for symbol" is much more informative when inspecting D
runtime type information in gdb, which is the only place where you would
encounter references to this compiler-generated symbol.

gdb/testsuite/ChangeLog:

	* gdb.dlang/demangle.exp: Update for demangling changes.