AST: Rename old form of {Generic,}FunctionType::get() to getOld()

This makes it easier to grep for and eventually remove the
remaining usages.

It also allows you to write FunctionType::get({}, ...) to call the
ArrayRef overload empty parameter list, instead of picking the Type
overload and calling it with an empty Type() value.

While I"m at it, in a few places instead of renaming just clean up
usages where it was completely mechanical to do so.

lib/Sema/CSSimplify.cpp

@@ -2603,7 +2603,7 @@ ConstraintSystem::simplifyConstructionConstraint(
   // constraint itself.
   addValueMemberConstraint(MetatypeType::get(valueType, TC.Context),
                            DeclBaseName::createConstructor(),
-                           FunctionType::get(tv, resultType),
+                           FunctionType::getOld(tv, resultType),
                            useDC, functionRefKind,
                            /*outerAlternatives=*/{},
                            getConstraintLocator(