Fix predefine for __NSConstantString struct type

Per review feedback the name was wrong and it can be used outside
Objective-C.

Unfortunately, making the internal struct visible broke some ASTMatchers
tests that assumed that the first record decl would be from user code,
rather than a builtin type.  I'm worried that this will also affect
users' code.  So this patch adds a typedef to wrap the internal struct
and only makes the typedef visible to namelookup.  This is sufficient to
allow the ASTReader to merge the decls we need without making the struct
itself visible.

rdar://problem/24425801

llvm-svn: 259734