Bridged specialized uses of Objective-C generic collections to more-specialized Swift collections.

When importing a specialized type for an Objective-C collection (e.g.,
NSArray<NSString *> *) that is being bridged to a Swift collection,
produce a more specialized Swift collection type (e.g., [String]) that
we would for the unspecialized type ([NSObject]).

The CMake and lit hackery is here because we need to be able to build
against versions of Clang that both do and do not have Objective-C
generics.

Swift SVN r24196

test/Inputs/objc-generics-sdk/usr/include/objc/objc.h

@@ -0,0 +1,31 @@
+#ifndef OBJC_OBJC_H_
+#define OBJC_OBJC_H_
+
+#define OBJC_ARC_UNAVAILABLE __attribute__((unavailable("not available in automatic reference counting mode")))
+#define NS_AUTOMATED_REFCOUNT_UNAVAILABLE OBJC_ARC_UNAVAILABLE
+
+typedef unsigned long NSUInteger;
+typedef long NSInteger;
+typedef __typeof__(__objc_yes) BOOL;
+
+typedef struct objc_selector    *SEL;
+SEL sel_registerName(const char *str);
+
+void NSDeallocateObject(id object) NS_AUTOMATED_REFCOUNT_UNAVAILABLE;
+
+#undef NS_AUTOMATED_REFCOUNT_UNAVAILABLE
+
+#define OBJC_ENUM(_type, _name) enum _name : _type _name; enum _name : _type
+#define OBJC_OPTIONS(_type, _name) enum _name : _type _name; enum _name : _type
+
+typedef OBJC_ENUM(int, objc_abi) {
+  objc_v1 = 0,
+  objc_v2 = 2
+};
+
+typedef OBJC_OPTIONS(int, objc_flags) {
+  objc_taggedPointer = 1 << 0,
+  objc_swiftRefcount = 1 << 1
+};
+
+#endif