libiberty: Add support for demangling local D template declarations

The D language now allows multiple different template declarations in
the same function that have the same mangled name.  To make the mangled
names unique, a fake parent in the form `__Sddd' is added to the symbol.
This information is not important for the user, so the demangler now
handles and ignores it.

libiberty/ChangeLog:

	* d-demangle.c (dlang_identifier): Skip over fake parent manglings.
	* testsuite/d-demangle-expected: Add tests.