[clang-tidy] Reduce the dependencies for the "make-confusable-table" tool

When cross compiling llvm, a separate recursive native cmake build
is generated, for building the tools that generate code (unless they're
provided externally by the caller).

This reduces the number of build steps for that native build from
1000+ steps to 162.

This matches how the clang-pseudo-gen tool is set up in
clang-tools-extra/pseudo/gen/CMakeLists.txt.

Differential Revision: https://reviews.llvm.org/D129797