[pseudo] Split greatergreater token.

For a >> token (a right shift operator, or a nested template?), the clang
lexer always returns a single greatergreater token, as a result,
the grammar-based GLR parser never try to parse the nested template
case.

We derive a token stream by always splitting the >> token, so that the
GLR parser is able to pursue both options during parsing (usually 1
path fails).

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D121678