[clang][Sema] close IsStandardConversion hole when adding cfi_unchecked_callee

Commit b194cf1 changed this function for the case where attribute
cfi_unchecked_callee is added in a function conversion. But this
introduces a hole (issue #162798), and it seems the change was
unnecessary: the preceding TryFunctionConversion will already allow
adding the cfi_unchecked_callee attribute, and will update FromType if
it succeeds. So we revert the changes to IsStandardConversion.

Fixes: #162798

Author: brunodf-snps

clang/lib/Sema/SemaOverload.cpp

@@ -2532,15 +2532,12 @@ static bool IsStandardConversion(Sema &S, Expr* From, QualType ToType,
 
   SCS.setToType(2, FromType);
 
-  // If we have not converted the argument type to the parameter type,
-  // this is a bad conversion sequence, unless we're resolving an overload in C.
-  //
-  // Permit conversions from a function without `cfi_unchecked_callee` to a
-  // function with `cfi_unchecked_callee`.
-  if (CanonFrom == CanonTo || S.AddingCFIUncheckedCallee(CanonFrom, CanonTo))
+  if (CanonFrom == CanonTo)
     return true;
 
-  if ((S.getLangOpts().CPlusPlus || !InOverloadResolution))
+  // If we have not converted the argument type to the parameter type,
+  // this is a bad conversion sequence, unless we're resolving an overload in C.
+  if (S.getLangOpts().CPlusPlus || !InOverloadResolution)
     return false;
 
   ExprResult ER = ExprResult{From};

clang/test/Frontend/cfi-unchecked-callee-attribute.cpp

@@ -9,6 +9,7 @@ void (*checked_ptr)(void) = unchecked;  // expected-warning{{implicit conversion
 void (CFI_UNCHECKED_CALLEE *unchecked_ptr)(void) = unchecked;
 void (CFI_UNCHECKED_CALLEE *from_normal)(void) = checked;
 void (CFI_UNCHECKED_CALLEE *c_no_function_decay)(void) = &unchecked;
+void (CFI_UNCHECKED_CALLEE __attribute__((noreturn)) *other_conflict)(void) = &checked; // expected-error{{cannot initialize a variable of type 'void (*)() __attribute__((noreturn)) __attribute__((cfi_unchecked_callee))' with an rvalue of type 'void (*)()'}}
 void (CFI_UNCHECKED_CALLEE *arr[10])(void);
 void (*cfi_elem)(void) = arr[1];  // expected-warning{{implicit conversion from 'void (*)() __attribute__((cfi_unchecked_callee))' to 'void (*)()' discards 'cfi_unchecked_callee' attribute}}
 void (CFI_UNCHECKED_CALLEE *cfi_unchecked_elem)(void) = arr[1];