[clang][Sema] Add flag to LookupName to force C/ObjC codepath

[bulbazord]

Motivation: The intent here is for use in Swift.
When building a clang module for swift consumption, swift adds an
extension block to the module for name lookup purposes. Swift calls
this a SwiftLookupTable. One purpose that this serves is to handle
conflicting names between ObjC classes and ObjC protocols. They exist in
different namespaces in ObjC programs, but in Swift they would exist in
the same namespace. Swift handles this by appending a suffix to a
protocol name if it shares a name with a class. For example, if you have
an ObjC class named "Foo" and a protocol with the same name, the
protocol would be renamed to "FooProtocol" when imported into swift.

When constructing the previously mentioned SwiftLookupTable, we use
Sema::LookupName to look up name conflicts for the previous problem.
By this time, the Parser has long finished its job so the call to
LookupName gets nullptr for its Scope (TUScope will be nullptr
by this point). The C/ObjC path does not have this problem because it
only uses the Scope in specific scenarios. The C++ codepath uses the
Scope quite extensively and will fail early on if the Scope it gets is
null. In our very specific case of looking up ObjC classes with a
specific name, we want to force sema::LookupName to take the C/ObjC
codepath even if C++ or ObjC++ is enabled.

(cherry picked from commit c57f034)

[hyp]

@swift-ci please test