[NFC][CLANG] Fix static code analyzer concerns

Reported by Static Code Analyzer Tool:

Inside "CGExprConstant.cpp" file, VisitObjCEncodeExpr() returns null value which is dereferenced without checking.

This patch adds an assert.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D151280

clang/lib/CodeGen/CGExprConstant.cpp

@@ -1340,6 +1340,7 @@ class ConstExprEmitter :
     std::string Str;
     CGM.getContext().getObjCEncodingForType(E->getEncodedType(), Str);
     const ConstantArrayType *CAT = CGM.getContext().getAsConstantArrayType(T);
+    assert(CAT && "String data not of constant array type!");
 
     // Resize the string to the right size, adding zeros at the end, or
     // truncating as needed.