Instantiation of a CXXMethodDecl may fail when the parameter type cannot be...

Instantiation of a CXXMethodDecl may fail when the parameter type cannot be instantiated. Do not crash in this case. Fixes PR22040!

The FIXME in the test is caused by TemplateDeclInstantiator::VisitCXXRecordDecl
returning a nullptr instead of creating an invalid decl. This is a common
pattern across all of TemplateDeclInstantiator, so I'm not comfortable changing
it. The reason it's not invalid in the class template is due to support for an
MSVC extension, see r137573.

llvm-svn: 225071