[Type checker] Eliminate TypeCheckExprFlags::DisableStructuralChecks.

This is always set along with SubExpressionDiagnostics, so use that
for both purposes. That flag will go away when `CSDiag.cpp` goes away.

Author: DougGregor

lib/Sema/CSDiag.cpp

@@ -394,15 +394,10 @@ Expr *FailureDiagnosis::typeCheckChildIndependently(
   // type check operation.
   Expr *preCheckedExpr = subExpr;
 
-  // Disable structural checks, because we know that the overall expression
-  // has type constraint problems, and we don't want to know about any
-  // syntactic issues in a well-typed subexpression (which might be because
-  // the context is missing).
-  TypeCheckExprOptions TCEOptions = TypeCheckExprFlags::DisableStructuralChecks;
-
   // Make sure that typechecker knows that this is an attempt
   // to diagnose a problem.
-  TCEOptions |= TypeCheckExprFlags::SubExpressionDiagnostics;
+  TypeCheckExprOptions TCEOptions =
+      TypeCheckExprFlags::SubExpressionDiagnostics;
 
   // Claim that the result is discarded to preserve the lvalue type of
   // the expression.

lib/Sema/TypeCheckConstraints.cpp

@@ -2240,7 +2240,7 @@ Type TypeChecker::typeCheckExpression(Expr *&expr, DeclContext *dc,
   // Unless the client has disabled them, perform syntactic checks on the
   // expression now.
   if (!cs.shouldSuppressDiagnostics() &&
-      !options.contains(TypeCheckExprFlags::DisableStructuralChecks)) {
+      !options.contains(TypeCheckExprFlags::SubExpressionDiagnostics)) {
     bool isExprStmt = options.contains(TypeCheckExprFlags::IsExprStmt);
     performSyntacticExprDiagnostics(result, dc, isExprStmt);
   }

lib/Sema/TypeChecker.h

@@ -163,10 +163,6 @@ enum class TypeCheckExprFlags {
   /// disables constraints forcing an lvalue result to be loadable.
   IsDiscarded = 0x01,
 
-  /// Whether the client wants to disable the structural syntactic restrictions
-  /// that we force for style or other reasons.
-  DisableStructuralChecks = 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