[CGExprConstant] stop calling into ConstExprEmitter for Reference type destinations (#70366)

Fixes a bug introduced by
commit b54294e ("[clang][ConstantEmitter] have
tryEmitPrivate[ForVarInit] try ConstExprEmitter fast-path first")

In the added test case, the QualType is a LValueReferenceType.

    LValueReferenceType 0x558412998d90 'const char (&)[41]'
    `-ParenType 0x558412998d30 'const char[41]' sugar
      `-ConstantArrayType 0x558412998cf0 'const char[41]' 41
        `-QualType 0x55841294c271 'const char' const
          `-BuiltinType 0x55841294c270 'char'

Fixes: #69979
(cherry picked from commit d9b15b0)

Author: nickdesaulniers

clang/lib/CodeGen/CGExprConstant.cpp

@@ -1736,9 +1736,10 @@ llvm::Constant *ConstantEmitter::tryEmitPrivate(const Expr *E,
                                                 QualType destType) {
   assert(!destType->isVoidType() && "can't emit a void constant");
 
-  if (llvm::Constant *C =
-          ConstExprEmitter(*this).Visit(const_cast<Expr *>(E), destType))
-    return C;
+  if (!destType->isReferenceType())
+    if (llvm::Constant *C =
+            ConstExprEmitter(*this).Visit(const_cast<Expr *>(E), destType))
+      return C;
 
   Expr::EvalResult Result;
 

clang/test/CodeGenCXX/const-init-cxx11.cpp

@@ -424,6 +424,8 @@ namespace DR2126 {
 // CHECK: @_ZN33ClassTemplateWithStaticDataMember3useE ={{.*}} constant ptr @_ZGRN33ClassTemplateWithStaticDataMember1SIvE1aE_
 // CHECK: @_ZGRN39ClassTemplateWithHiddenStaticDataMember1SIvE1aE_ = linkonce_odr hidden constant i32 5, comdat
 // CHECK: @_ZN39ClassTemplateWithHiddenStaticDataMember3useE ={{.*}} constant ptr @_ZGRN39ClassTemplateWithHiddenStaticDataMember1SIvE1aE_
+// CHECK: @.str.[[STR:[0-9]+]] ={{.*}} constant [9 x i8] c"12345678\00"
+// CHECK-NEXT: @e = global %struct.PR69979 { ptr @.str.[[STR]] }
 // CHECK: @_ZGRZN20InlineStaticConstRef3funEvE1i_ = linkonce_odr constant i32 10, comdat
 // CHECK20: @_ZZN12LocalVarInit4dtorEvE1a = internal constant {{.*}} i32 103
 
@@ -632,6 +634,10 @@ struct X {
 const char *f() { return &X::p; }
 }
 
+struct PR69979 {
+  const char (&d)[9];
+} e {"12345678"};
+
 // VirtualMembers::TemplateClass::templateMethod() must be defined in this TU,
 // not just declared.
 // CHECK: define linkonce_odr void @_ZN14VirtualMembers13TemplateClassIiE14templateMethodEv(ptr {{[^,]*}} %this)