[clang][ExprConst] Allow comparisons with string literals (#106733)

Don't diagnose them, but literals still have distinct addresses.

Fixes #58754

Author: tbaederr

clang/lib/AST/ExprConstant.cpp

@@ -2146,7 +2146,7 @@ static bool IsLiteralLValue(const LValue &Value) {
   if (Value.getLValueCallIndex())
     return false;
   const Expr *E = Value.Base.dyn_cast<const Expr*>();
-  return E && !isa<MaterializeTemporaryExpr>(E);
+  return E && !isa<MaterializeTemporaryExpr, StringLiteral>(E);
 }
 
 static bool IsWeakLValue(const LValue &Value) {

clang/test/AST/ByteCode/cxx20.cpp

@@ -108,22 +108,16 @@ constexpr auto p2 = "test2";
 constexpr bool b1 = foo(p1) == foo(p1);
 static_assert(b1);
 
-constexpr bool b2 = foo(p1) == foo(p2); // ref-error {{must be initialized by a constant expression}} \
-                                        // ref-note {{comparison of addresses of literals}} \
-                                        // ref-note {{declared here}}
-static_assert(!b2); // ref-error {{not an integral constant expression}} \
-                    // ref-note {{not a constant expression}}
+constexpr bool b2 = foo(p1) == foo(p2);
+static_assert(!b2);
 
 constexpr auto name1() { return "name1"; }
 constexpr auto name2() { return "name2"; }
 
 constexpr auto b3 = name1() == name1();
 static_assert(b3);
-constexpr auto b4 = name1() == name2(); // ref-error {{must be initialized by a constant expression}} \
-                                        // ref-note {{has unspecified value}} \
-                                        // ref-note {{declared here}}
-static_assert(!b4); // ref-error {{not an integral constant expression}} \
-                    // ref-note {{not a constant expression}}
+constexpr auto b4 = name1() == name2();
+static_assert(!b4);
 
 namespace UninitializedFields {
   class A {

clang/test/SemaCXX/constant-expression-cxx11.cpp

@@ -358,11 +358,9 @@ struct Str {
 
 extern char externalvar[];
 constexpr bool constaddress = (void *)externalvar == (void *)0x4000UL; // expected-error {{must be initialized by a constant expression}} expected-note {{reinterpret_cast}}
-constexpr bool litaddress = "foo" == "foo"; // expected-error {{must be initialized by a constant expression}}
-// expected-note@-1 {{comparison of addresses of literals has unspecified value}}
-// cxx20_23-warning@-2 {{comparison between two arrays is deprecated}}
+constexpr bool litaddress = "foo" == "foo"; // cxx20_23-warning {{comparison between two arrays is deprecated}}
 static_assert(0 != "foo", "");
-
+static_assert("foo" != "foo", "");// cxx20_23-warning {{comparison between two arrays is deprecated}}
 }
 
 namespace MaterializeTemporary {