[clang-tidy] Add CLANG_TIDY_CONFUSABLE_CHARS_GEN cmake cache variable to avoid...

[clang-tidy] Add CLANG_TIDY_CONFUSABLE_CHARS_GEN cmake cache variable to avoid building when cross compiling

This is similar to the LLVM_TABLEGEN, CLANG_TABLEGEN and
CLANG_PSEUDO_GEN cmake cache variables.

Differential Revision: https://reviews.llvm.org/D129799