c++: lambda mangling alias issues [PR107897]

In 107897, by the time we are looking at the mangling clash, the
alias has already been removed from the symbol table by analyze_functions,
so we can't look at n->cpp_implicit_alias.  So just assume that it's an
alias if it's internal.

In 108887 the problem is that removing the mangling alias from the symbol
table confuses analyze_functions, because it ended up as first_analyzed
somehow, so it becomes a dangling pointer.  So instead we call reset()
to neutralize the alias.  To make this work for variables, I needed to move
reset() from cgraph_node to symtab_node.

	PR c++/107897
	PR c++/108887

gcc/ChangeLog:

	* cgraph.h: Move reset() from cgraph_node to symtab_node.
	* cgraphunit.cc (symtab_node::reset): Adjust.  Also call
	remove_from_same_comdat_group.

gcc/cp/ChangeLog:

	* decl2.cc (record_mangling): Use symtab_node::reset.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-lambda3.C: Use -flto if supported.
	* g++.dg/cpp0x/lambda/lambda-mangle7.C: New test.