[Sema] check InitListExpr format strings like {"foo"}

Adds InitListExpr case in format string checks.

e.g.

    int sprintf(char *__restrict, const char * __restrict, ...);

    int foo()
    {
        char data[100];
        constexpr const char* fmt2{"%d"};  // no-warning
        sprintf(data, fmt2, 123);
    }

Fixes: https://github.com/llvm/llvm-project/issues/58900

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D137839

clang/lib/Sema/SemaChecking.cpp

@@ -8585,6 +8585,15 @@ tryAgain:
     return SLCT_UncheckedLiteral;
 
   switch (E->getStmtClass()) {
+  case Stmt::InitListExprClass:
+    // Handle expressions like {"foobar"}.
+    if (const clang::Expr *SLE = maybeConstEvalStringLiteral(S.Context, E)) {
+      return checkFormatStringExpr(S, SLE, Args, APK, format_idx, firstDataArg,
+                                   Type, CallType, /*InFunctionCall*/ false,
+                                   CheckedVarArgs, UncoveredArg, Offset,
+                                   IgnoreStringsWithoutSpecifiers);
+    }
+    return SLCT_NotALiteral;
   case Stmt::BinaryConditionalOperatorClass:
   case Stmt::ConditionalOperatorClass: {
     // The expression is a literal if both sub-expressions were, and it was

clang/test/SemaCXX/format-strings.cpp

@@ -203,6 +203,12 @@ void f() {
   printf(string_linebreak(), 1, 2, 3, 4); // expected-warning {{format specifies type 'char *' but the argument has type 'int'}}
   printf(not_literal(), 1, 2, 3, 4); // expected-warning {{format string is not a string literal}}
   printf(wrap_constexpr(), 1, 2); // expected-warning {{format specifies type 'char *' but the argument has type 'int'}}
+
+  constexpr const char *fmt {"%d%d"};
+  printf(fmt, 1, 1); // no-warning
+
+  constexpr const char *fmt2 {"%d"}; // expected-note{{format string is defined here}}
+  printf(fmt2, "oops"); // expected-warning{{format specifies type 'int' but the argument has type}}
 }