[clang-tidy] Rename the make-confusable-table executable

Rename it to clang-tidy-confusable-chars-gen, to make its role
clearer in a wider context.

In cross builds, the caller might want to provide this tool
externally (to avoid needing to rebuild it in the cross build).
In such a case, having the tool properly namespaced makes its role
clearer.

This matches how the clang-pseudo-gen tool was renamed in
a43fef05 / D126725.

Differential Revision: https://reviews.llvm.org/D129798