[CSSimplify] Fix warning check to account that "from" might be less optional than "to" type

While checking whether compiler needs to produce a checked cast warning,
account for the fact that "from" could be less optional than "to" e.g.
`0 as Any?`, so the difference has to be stored as a signed integer
otherwise it's going to underflow and result in a crash or infinite
recursion in the diagnostics.

Resolves: rdar://79523605

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -6549,7 +6549,9 @@ static ConstraintFix *maybeWarnAboutExtraneousCast(
   if (!castExpr)
     return nullptr;
 
-  unsigned extraOptionals = fromOptionals.size() - toOptionals.size();
+  // "from" could be less optional than "to" e.g. `0 as Any?`, so
+  // we need to store the difference as a signed integer.
+  int extraOptionals = fromOptionals.size() - toOptionals.size();
   // Removing the optionality from to type when the force cast expr is an IUO.
   const auto *const TR = castExpr->getCastTypeRepr();
   if (isExpr<ForcedCheckedCastExpr>(anchor) && TR &&

validation-test/Sema/type_checker_crashers_fixed/rdar79523605.swift

@@ -0,0 +1,5 @@
+// RUN: %target-swift-frontend %s -typecheck -verify
+
+func test() -> Any? {
+  0 as! Any? // expected-warning {{forced cast from 'Int' to 'Any?' always succeeds; did you mean to use 'as'?}}
+}