[NFC] Refactor PasteTokens so that it can be passed the Token Stream and Index...

[NFC] Refactor PasteTokens so that it can be passed the Token Stream and Index to start concatenating at.
  In passing:
    - change the name of the function to pasteTokens c/w coding standards
    - rename CurToken to CurTokenIdx (since it is not the token, but the index)
    - add doxygen comments to document some of pasteTokens' functionality
    - use parameter names different from the data member names.

This will be useful for implementing __VA_OPT__ (https://reviews.llvm.org/D35782#inline-322587)

llvm-svn: 314747