[Flang] Use find_program() to find clang-tblgen

Use find_program() to find the clang-tblgen executable. The current
code ends up using docs/clang-tblgen for me...

This is the same way that mlir-tblgen is obtained in:
https://github.com/llvm/llvm-project/blob/27241435515554bc21105713c9d34cf886c5bced/flang/CMakeLists.txt#L95-L96.

Differential Revision: https://reviews.llvm.org/D131475