[Clang importer] Don't bridge blocks to Swift functions in ObjC generic args.

While the compiler can bridge C block types to Swift function types,
the Swift runtime cannot. Don't bridge block types to Swift function
types in Objective-C generic arguments, so

  NSArray<some-block-type>

will get imported as

  [@convention(block) (...) -> Whatever]

rather than

  [(...) -> Whatever]

Fixes rdar://problem/40879067 in a fairly narrow way; the Clang
importer's approach to adjusting types based on context needs a
cleanup, but this is the safe, localized fix suitable for 4.2.

test/Inputs/clang-importer-sdk/usr/include/objc_generics.h

@@ -113,3 +113,10 @@ typedef id <Fungible> FungibleObject;
 
 @interface Third : Second<Third *>
 @end
+
+typedef void (^ _Nonnull BlockPointerType)(void);
+
+@interface HasBlockArray : NSObject
+- (NSArray<BlockPointerType> * _Nonnull)blockArray;
+- (BlockPointerType)blockPointerType;
+@end