Reclaim "in" as an identifier

In Swift the "in" keyword is really a form of punctuation, and highly
context specific punctuation at that. It never begins a statement, nor
does the grammar require it be statement keyword. The grammar also
doesn't use it outside of for-each loops, and its use within a for-each
loop is highly unambiguous.

Thanks to Chris for the performance related feedback. This improves the
performance of getter/setter parsing as well.

Swift SVN r3880

include/swift/Parse/Token.h

@@ -75,7 +75,11 @@ public:
   bool isNotAnyOperator() const {
     return !isAnyOperator();
   }
-  
+
+  bool isContextualKeyword(StringRef ContextKW) const {
+    return is(tok::identifier) && Text == ContextKW;
+  }
+
   bool isAnyLParen() const {
     return Kind == tok::l_paren || Kind == tok::l_paren_call;
   }

include/swift/Parse/Tokens.def

@@ -58,7 +58,6 @@ KEYWORD(subscript)
 
 // Statement keywords.
 KEYWORD(if)
-KEYWORD(in)
 KEYWORD(do)
 KEYWORD(else)
 KEYWORD(for)