[clang][syntax] Replace `std::vector<bool>` use

LLVM Programmer’s Manual strongly discourages the use of `std::vector<bool>` and suggests `llvm::BitVector` as a possible replacement.

This patch replaces `std::vector<bool>` with `llvm::BitVector` in the Syntax library and replaces range-based for loop with regular for loop. This is necessary due to `llvm::BitVector` not having `begin()` and `end()` (D117116).

Reviewed By: dexonsmith, dblaikie

Differential Revision: https://reviews.llvm.org/D118109