[clang-format][NFC] Store FormatToken::Type as an enum instead of bitfield

This improves the debugging experience since LLDB will print the enumerator
name instead of a decimal number. This changes TokenType to have uint8_t
as the underlying type and moves it after the remaining bitfields to avoid
increasing the size of FormatToken.

Reviewed By: MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D87006