Objective-C++11: Support static_assert() in @interface/@implementation ivar...

Objective-C++11: Support static_assert() in @interface/@implementation ivar lists and method declarations

This adds support for static_assert() (and _Static_assert()) in
@interface/@implementation ivar lists and in @interface method declarations.

It was already supported in @implementation blocks outside of the ivar lists.

The assert AST nodes are added at file scope, matching where other
(non-Objective-C) declarations at @interface / @implementation level go (cf
`allTUVariables`).

Also add a `__has_feature(objc_c_static_assert)` that's true in C11 (and
`__has_extension(objc_c_static_assert)` that's always true) and
`__has_feature(objc_cxx_static_assert)` that's true in C++11 modea fter this
patch, so it's possible to check if this is supported.

Differential Revision: https://reviews.llvm.org/D59223

llvm-svn: 356148