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

There are two scenarios here:

1. Standalone flang build, where we use an installed clang-tblgen
   binary. We need to use find_package() to find it.
2. Combined build of clang and flang, where we want to use the
   path specified in CLANG_TABLEGEN_EXE during the clang build --
   however, this variable was previously not exported.

The new implementation matches what is done for mlir-tblgen.

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