Implement syntax changes for in-place methods.

Rename 'assignment' attribute of infix operators to 'mutating'. Add
'has_assignment' attribute, which results in an implicit declaration of
the assignment version of the same operator. Parse "func =foo"
declaration and "foo.=bar" expression. Validate some basic properties of
in-place methods.

Not yet implemented: automatic generation of wrapper for =foo() if foo()
is implemented, or vice versa; likewise for operators.

Swift SVN r26508

include/swift/Serialization/ModuleFormat.h

@@ -895,10 +895,10 @@ namespace decls_block {
     DeclContextIDField,// context decl
     AssociativityField,
     BCFixed<8>,  // precedence
-    BCFixed<1>,  // assignment
+    BCFixed<1>,  // mutating
     BCFixed<1>,  // IsAssocImplicit flag
     BCFixed<1>,  // IsPrecedenceImplicit flag
-    BCFixed<1>   // IsAssignmentImplicit flag
+    BCFixed<1>   // IsMutatingImplicit flag
   >;
 
   using EnumElementLayout = BCRecordLayout<