Start parsing 'throws' on initializers.

Introduce basic validation for throwing @objc initializers, e.g., a
failable @objc initializer cannot also be throwing. However,
Objective-C selector computation is broken.

Swift SVN r27292

include/swift/AST/Decl.h

@@ -5275,6 +5275,9 @@ class ConstructorDecl : public AbstractFunctionDecl {
   /// The location of the '!' or '?' for a failable initializer.
   SourceLoc FailabilityLoc;
 
+  // Location of the 'throws' token.
+  SourceLoc ThrowsLoc;
+
   Pattern *BodyParams[2];
 
   /// The type of the initializing constructor.
@@ -5295,7 +5298,8 @@ public:
   ConstructorDecl(DeclName Name, SourceLoc ConstructorLoc, 
                   OptionalTypeKind Failability, SourceLoc FailabilityLoc,
                   Pattern *SelfBodyParam, Pattern *BodyParams,
-                  GenericParamList *GenericParams, DeclContext *Parent);
+                  GenericParamList *GenericParams, 
+                  SourceLoc throwsLoc, DeclContext *Parent);
 
   void setBodyParams(Pattern *selfPattern, Pattern *bodyParams);
 
@@ -5417,6 +5421,9 @@ public:
   /// Retrieve the location of the '!' or '?' in a failable initializer.
   SourceLoc getFailabilityLoc() const { return FailabilityLoc; }
 
+  /// Retrieve the location of the 'throws' keyword, if present.
+  SourceLoc getThrowsLoc() const { return ThrowsLoc; }
+
   /// Whether the implementation of this method is a stub that traps at runtime.
   bool hasStubImplementation() const {
     return ConstructorDeclBits.HasStubImplementation;

include/swift/AST/DiagnosticsSema.def

@@ -1918,6 +1918,10 @@ ERROR(objc_invalid_on_throwing_result,sema_objc,none,
       "it returns a value of type %1; return 'Void' or a type that bridges "
       "to an Objective-C class",
       (unsigned, Type))
+ERROR(objc_invalid_on_failing_init,sema_objc,none,
+      "a failable and throwing initializer cannot be marked "
+      "%" OBJC_ATTR_SELECT "0 because 'nil' indicates failure to Objective-C",
+      (unsigned))
 
 #undef OBJC_ATTR_SELECT