Parsing, semantic analysis, and AST for Objective-C type parameters.

Parsing, semantic analysis, and AST for Objective-C type parameters.

Produce type parameter declarations for Objective-C type parameters,
and attach lists of type parameters to Objective-C classes,
categories, forward declarations, and extensions as
appropriate. Perform semantic analysis of type bounds for type
parameters, both in isolation and across classes/categories/extensions
to ensure consistency.

Also handle (de-)serialization of Objective-C type parameter lists,
along with sundry other things one must do to add a new declaration to
Clang.

Note that Objective-C type parameters are typedef name declarations,
like typedefs and C++11 type aliases, in support of type erasure.

Part of rdar://problem/6294649.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241541 91177308-0d34-0410-b5e6-96231b3b80d8