[CodeGen][ObjC] Do not call objc_storeStrong when initializing a

[CodeGen][ObjC] Do not call objc_storeStrong when initializing a
constexpr variable.

When compiling a constexpr NSString initialized with an objective-c
string literal, CodeGen emits objc_storeStrong on an uninitialized
alloca, which causes a crash.

This patch folds the code in EmitScalarInit into EmitStoreThroughLValue
and fixes the crash by calling objc_retain on the string instead of
using objc_storeStrong.

rdar://problem/28562009

Differential Revision: https://reviews.llvm.org/D25547

llvm-svn: 284516