[ObjC][CodeGen] Remove an assert that is no longer correct.

clang asserts when compiling the following code because r231508 made
changes to promote constant temporary arrays and records to globals
with constant initializers:

std::vector<NSString*> strs = {@"a", @"b"};

This commit changes the code to return early if the object returned by
createReferenceTemporary is a global variable with an initializer.

rdar://problem/25504992
rdar://problem/25955179

Differential Revision: http://reviews.llvm.org/D20045

llvm-svn: 269385