[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
diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 94823ae..aa75ea3 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -1629,11 +1629,19 @@
       break;
 
     case Qualifiers::OCL_Strong:
+      if (isInit) {
+        Src = RValue::get(EmitARCRetain(Dst.getType(), Src.getScalarVal()));
+        break;
+      }
       EmitARCStoreStrong(Dst, Src.getScalarVal(), /*ignore*/ true);
       return;
 
     case Qualifiers::OCL_Weak:
-      EmitARCStoreWeak(Dst.getAddress(), Src.getScalarVal(), /*ignore*/ true);
+      if (isInit)
+        // Initialize and then skip the primitive store.
+        EmitARCInitWeak(Dst.getAddress(), Src.getScalarVal());
+      else
+        EmitARCStoreWeak(Dst.getAddress(), Src.getScalarVal(), /*ignore*/ true);
       return;
 
     case Qualifiers::OCL_Autoreleasing: