[OBJC] Add attribute to mark Objective C class as non-lazy

A non-lazy class will be initialized eagerly when the Objective-C runtime is
loaded. This is required for certain system classes which have instances allocated in
non-standard ways, such as the classes for blocks and constant strings.
Adding this attribute is essentially equivalent to providing a trivial
+load method but avoids the (fairly small) load-time overheads associated
with defining and calling such a method.

Differential Revision: https://reviews.llvm.org/D56555

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@353116 91177308-0d34-0410-b5e6-96231b3b80d8