[c++20] Add deprecation warnings for the expression forms deprecated by P1120R0.

This covers:
 * usual arithmetic conversions (comparisons, arithmetic, conditionals)
   between different enumeration types
 * usual arithmetic conversions between enums and floating-point types
 * comparisons between two operands of array type

The deprecation warnings are on-by-default (in C++20 compilations); it
seems likely that these forms will become ill-formed in C++23, so
warning on them now by default seems wise.

For the first two bullets, off-by-default warnings were also added for
all the cases where we didn't already have warnings (covering language
modes prior to C++20). These warnings are in subgroups of the existing
-Wenum-conversion (except that the first case is not warned on if either
enumeration type is anonymous, consistent with our existing
-Wenum-conversion warnings).
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 9e5e49f..47b58df 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -5993,7 +5993,8 @@
   //      the usual arithmetic conversions are performed to bring them to a
   //      common type, and the result is of that type.
   if (LTy->isArithmeticType() && RTy->isArithmeticType()) {
-    QualType ResTy = UsualArithmeticConversions(LHS, RHS);
+    QualType ResTy =
+        UsualArithmeticConversions(LHS, RHS, QuestionLoc, ACK_Conditional);
     if (LHS.isInvalid() || RHS.isInvalid())
       return QualType();
     if (ResTy.isNull()) {