Move "clang/Basic/TokenKinds.h" into a separate top-level module.

Fixes modular build for clangPseudoGrammar from clang-tools-extra.

Starting from https://reviews.llvm.org/D126731 clangPseudoGrammar
doesn't depend on generated .inc headers but still depends on
"Basic/TokenKinds.h". It means clangPseudoGrammar depends on module
'Clang_Basic' which does depend on generated .inc headers. To avoid
these coarse dependencies and extra build steps, extract
"clang/Basic/TokenKinds.h" into a top-level module 'Clang_Basic_TokenKinds'.

rdar://97387951

Differential Revision: https://reviews.llvm.org/D130377