[CS] Remove AllowUnresolvedTypeVariables (swiftlang#84146)

This has been unused for a while now.

Author: hamishknight

include/swift/Sema/ConstraintSystem.h

@@ -1937,13 +1937,6 @@ enum class ConstraintSystemFlags {
   /// Set if the client wants diagnostics suppressed.
   SuppressDiagnostics = 0x02,
 
-  /// If set, the client wants a best-effort solution to the constraint system,
-  /// but can tolerate a solution where all of the constraints are solved, but
-  /// not all type variables have been determined.  In this case, the constraint
-  /// system is not applied to the expression AST, but the ConstraintSystem is
-  /// left in-tact.
-  AllowUnresolvedTypeVariables = 0x04,
-
   /// If set, verbose output is enabled for this constraint system.
   ///
   /// Note that this flag is automatically applied to all constraint systems,
@@ -1952,12 +1945,12 @@ enum class ConstraintSystemFlags {
   /// \c DebugConstraintSolverAttempt. Finally, it can also be automatically
   /// enabled for a pre-configured set of expressions on line numbers by setting
   /// \c DebugConstraintSolverOnLines.
-  DebugConstraints = 0x08,
+  DebugConstraints = 0x04,
 
   /// If set, we are solving specifically to determine the type of a
   /// CodeCompletionExpr, and should continue in the presence of errors wherever
   /// possible.
-  ForCodeCompletion = 0x10,
+  ForCodeCompletion = 0x08,
 
   /// Include Clang function types when checking equality for function types.
   ///
@@ -1968,16 +1961,16 @@ enum class ConstraintSystemFlags {
   /// should be treated as semantically different, as they may have different
   /// calling conventions, say due to Clang attributes such as
   /// `__attribute__((ns_consumed))`.
-  UseClangFunctionTypes = 0x20,
+  UseClangFunctionTypes = 0x10,
 
   /// When set, ignore async/sync mismatches
-  IgnoreAsyncSyncMismatch = 0x40,
+  IgnoreAsyncSyncMismatch = 0x20,
 
   /// Disable macro expansions.
-  DisableMacroExpansions = 0x80,
+  DisableMacroExpansions = 0x40,
 
   /// Enable old type-checker performance hacks.
-  EnablePerformanceHacks = 0x100,
+  EnablePerformanceHacks = 0x80,
 };
 
 /// Options that affect the constraint system as a whole.

lib/Sema/CSSolver.cpp

@@ -1488,16 +1488,6 @@ ConstraintSystem::solve(SyntacticElementTarget &target,
         return std::nullopt;
       }
 
-      if (Options.contains(
-            ConstraintSystemFlags::AllowUnresolvedTypeVariables)) {
-        dumpSolutions(solution);
-        auto ambiguousSolutions = std::move(solution).takeAmbiguousSolutions();
-        std::vector<Solution> result(
-            std::make_move_iterator(ambiguousSolutions.begin()),
-            std::make_move_iterator(ambiguousSolutions.end()));
-        return std::move(result);
-      }
-
       LLVM_FALLTHROUGH;
 
     case SolutionResult::UndiagnosedError:

unittests/Sema/BindingInferenceTests.cpp

@@ -22,10 +22,7 @@ using namespace swift::constraints;
 using namespace swift::constraints::inference;
 
 TEST_F(SemaTest, TestIntLiteralBindingInference) {
-  ConstraintSystemOptions options;
-  options |= ConstraintSystemFlags::AllowUnresolvedTypeVariables;
-
-  ConstraintSystem cs(DC, options);
+  ConstraintSystem cs(DC, std::nullopt);
 
   auto *intLiteral = IntegerLiteralExpr::createFromUnsigned(Context, 42, SourceLoc());