[clang][ConstExprEmitter] handle ArrayToPointerDecay ImplicitCastExpr of StringLiterals

Consider the following statement:
    const char* foo = "foo";

For the sub-AST:
    `-ImplicitCastExpr <col:19> 'const char *' <NoOp>
      `-ImplicitCastExpr <col:19> 'char *' <ArrayToPointerDecay>
        `-StringLiteral <col:19> 'char[4]' lvalue "foo"

The address of the StringLiteral can be emitted as the Constant.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D156185

Author: nickdesaulniers

clang/lib/CodeGen/CGExprConstant.cpp

@@ -1127,6 +1127,11 @@ class ConstExprEmitter :
     case CK_ConstructorConversion:
       return Visit(subExpr, destType);
 
+    case CK_ArrayToPointerDecay:
+      if (const auto *S = dyn_cast<StringLiteral>(subExpr))
+        return CGM.GetAddrOfConstantStringFromLiteral(S).getPointer();
+      return nullptr;
+
     case CK_IntToOCLSampler:
       llvm_unreachable("global sampler variables are not generated");
 
@@ -1164,7 +1169,6 @@ class ConstExprEmitter :
     case CK_CPointerToObjCPointerCast:
     case CK_BlockPointerToObjCPointerCast:
     case CK_AnyPointerToBlockPointerCast:
-    case CK_ArrayToPointerDecay:
     case CK_FunctionToPointerDecay:
     case CK_BaseToDerived:
     case CK_DerivedToBase: