[libSyntax] Fix parsing of key paths on specialised generics

Author: ahoppen

test/Syntax/Outputs/round_trip_parse_gen.swift.withkinds

@@ -446,6 +446,7 @@ func keypath<FunctionSignature><ParameterClause>() </ParameterClause></FunctionS
   _ </DiscardAssignmentExpr><AssignmentExpr>= </AssignmentExpr><KeyPathExpr>\<MemberAccessExpr><MemberAccessExpr><IdentifierExpr>a</IdentifierExpr>.b</MemberAccessExpr>.c</MemberAccessExpr></KeyPathExpr></SequenceExpr><SequenceExpr><DiscardAssignmentExpr>
   _ </DiscardAssignmentExpr><AssignmentExpr>= </AssignmentExpr><KeyPathExpr>\<SubscriptExpr><MemberAccessExpr><IdentifierExpr>a</IdentifierExpr>.b</MemberAccessExpr>[<FunctionCallArgument><IntegerLiteralExpr>1</IntegerLiteralExpr></FunctionCallArgument>]</SubscriptExpr></KeyPathExpr></SequenceExpr><SequenceExpr><DiscardAssignmentExpr>
   _ </DiscardAssignmentExpr><AssignmentExpr>= </AssignmentExpr><KeyPathExpr>\<MemberAccessExpr><MemberAccessExpr>.a</MemberAccessExpr>.b</MemberAccessExpr></KeyPathExpr></SequenceExpr><SequenceExpr><DiscardAssignmentExpr>
+  _ </DiscardAssignmentExpr><AssignmentExpr>= </AssignmentExpr><KeyPathExpr>\<SpecializeExpr><IdentifierExpr>Array</IdentifierExpr><GenericArgumentClause><<GenericArgument><SimpleTypeIdentifier>Int</SimpleTypeIdentifier></GenericArgument>></GenericArgumentClause></SpecializeExpr><SubscriptExpr><KeyPathBaseExpr>.</KeyPathBaseExpr>[]</SubscriptExpr></KeyPathExpr></SequenceExpr><SequenceExpr><DiscardAssignmentExpr>
   _ </DiscardAssignmentExpr><AssignmentExpr>= </AssignmentExpr><ObjcKeyPathExpr>#keyPath(<ObjcNamePiece>a.</ObjcNamePiece><ObjcNamePiece>b.</ObjcNamePiece><ObjcNamePiece>c</ObjcNamePiece>)</ObjcKeyPathExpr></SequenceExpr>
 }</CodeBlock></FunctionDecl><FunctionDecl>
 func objcSelector<FunctionSignature><ParameterClause>() </ParameterClause></FunctionSignature><CodeBlock>{<SequenceExpr><DiscardAssignmentExpr>

test/Syntax/round_trip_parse_gen.swift

@@ -446,6 +446,7 @@ func keypath() {
   _ = \a.b.c
   _ = \a.b[1]
   _ = \.a.b
+  _ = \Array<Int>.[]
   _ = #keyPath(a.b.c)
 }
 func objcSelector() {

utils/gyb_syntax_support/ExprNodes.py

@@ -498,7 +498,11 @@
     Node('KeyPathExpr', kind='Expr',
          children=[
              Child('Backslash', kind='BackslashToken'),
-             Child('RootExpr', kind='IdentifierExpr', is_optional=True), 
+             Child('RootExpr', kind='Expr', is_optional=True, 
+                   node_choices=[
+                       Child('IdentifierExpr', kind='IdentifierExpr'),
+                       Child('SpecializeExpr', kind='SpecializeExpr')
+                   ]), 
              Child('Expression', kind='Expr'),
          ]),