clang-format: [JS] detect C++ keywords.

Summary:
C++ defines a number of keywords that are regular identifiers in
JavaScript, e.g. `concept`:

    const concept = 1; // legit JS

This change expands the existing `IsJavaScriptIdentifier(Tok)` function
to return false for C++ keywords that aren't keywords in JS.

Reviewers: krasimir

Subscribers: jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77311