PR22117: Fix a case where we would get confused about which function parameter

we're instantiating, if there's a ParmVarDecl within a FunctionDecl context
that is not a parameter of that function. Add some asserts to catch this kind
of issue more generally, and fix another bug exposed by those asserts where we
were missing a local instantiation scope around substitution of
explicitly-specified template arguments.

llvm-svn: 225490