[CodeCompletion] Improve completion for Swift keypath expression

* Handle completion in 'parseExprKeyPath()' instead of
  'parseExprPostfixSuffix()'.
* Fix a crash for implicit type keypath. e.g. '\.path.<complete>'. (SR-8042).
* Use 'completeExprKeyPath()' callback.
* Implement completion without '.'. e.g. '\Ty.path<complete>'
* Improved handling for 'subscript' in completion.
* Improved handling for optional unwrapping in completion.

https://bugs.swift.org/browse/SR-8042
rdar://problem/41262612

include/swift/Parse/Parser.h

@@ -160,8 +160,6 @@ public:
   bool InPoundLineEnvironment = false;
   bool InPoundIfEnvironment = false;
   bool InSwiftKeyPath = false;
-  Expr* SwiftKeyPathRoot = nullptr;
-  SourceLoc SwiftKeyPathSlashLoc = SourceLoc();
 
   LocalContext *CurLocalContext = nullptr;