Improves overload completion result chunks.

The code building the code completion string for overloads was providing
less detail compared to the one building completion strings for function
declarations. There was no information about optionals and no information
about what's a parameter and what's a function identifier, everything
besides ResultType, CurrentParameter and special characters was classified
as Text.

This makes code completion strings for overload candidates to follow a
pattern very similar, but not identical, to the one in use for function
declarations:

 - return type chunk: ResultType
 - function identifier chunk: Text
 - parameter chunks: Placeholder
 - optional parameter chunks: Optional
 - current parameter chunk: CurrentParameter

llvm-svn: 227309