[ConstraintSystem] NFC: Warn if result of generate* is unused

Not checking success of constraint generation could lead to crashes
e.g. when follow-up code accesses something that hasn't been processed
due to error.

(cherry picked from commit b0f7aef)

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -4544,6 +4544,7 @@ class ConstraintSystem {
   /// Generate constraints for the given solution target.
   ///
   /// \returns true if an error occurred, false otherwise.
+  LLVM_NODISCARD
   bool generateConstraints(SolutionApplicationTarget &target,
                            FreeTypeVariableBinding allowFreeTypeVariables);
 
@@ -4552,18 +4553,21 @@ class ConstraintSystem {
   /// \param closure the closure expression
   ///
   /// \returns \c true if constraint generation failed, \c false otherwise
+  LLVM_NODISCARD
   bool generateConstraints(ClosureExpr *closure);
 
   /// Generate constraints for the given (unchecked) expression.
   ///
   /// \returns a possibly-sanitized expression, or null if an error occurred.
+  LLVM_NODISCARD
   Expr *generateConstraints(Expr *E, DeclContext *dc,
                             bool isInputExpression = true);
 
   /// Generate constraints for binding the given pattern to the
   /// value of the given expression.
   ///
   /// \returns a possibly-sanitized initializer, or null if an error occurred.
+  LLVM_NODISCARD
   Type generateConstraints(Pattern *P, ConstraintLocatorBuilder locator,
                            bool bindPatternVarsOneWay,
                            PatternBindingDecl *patternBinding,
@@ -4573,6 +4577,7 @@ class ConstraintSystem {
   ///
   /// \returns true if there was an error in constraint generation, false
   /// if generation succeeded.
+  LLVM_NODISCARD
   bool generateConstraints(StmtCondition condition, DeclContext *dc);
 
   /// Generate constraints for a case statement.
@@ -4582,6 +4587,7 @@ class ConstraintSystem {
   ///
   /// \returns true if there was an error in constraint generation, false
   /// if generation succeeded.
+  LLVM_NODISCARD
   bool generateConstraints(CaseStmt *caseStmt, DeclContext *dc,
                            Type subjectType, ConstraintLocator *locator);
 
@@ -4625,6 +4631,7 @@ class ConstraintSystem {
   /// \param propertyType The type of the wrapped property.
   ///
   /// \returns true if there is an error.
+  LLVM_NODISCARD
   bool generateWrappedPropertyTypeConstraints(VarDecl *wrappedVar,
                                               Type initializerType,
                                               Type propertyType);