libstdc++: Fix pretty printer lookup for class templates [PR122812]

Under some circumstances the type.name of a pair<> type starts with
"struct". This confuses GDB when we use gdb.lookup_type for the name of
template specialization using "struct pair<...>" in its template
argument list.

Using type.tag avoids this problem.

libstdc++-v3/ChangeLog:

	PR libstdc++/122812
	* python/libstdcxx/v6/printers.py (lookup_templ_spec): Use
	gdb.Type.tag if present.

(cherry picked from commit dce0d564d742617f71297c88e4a1c0d6038f2c47)
1 file changed