Split flang.exe into flang-driver

[ghost]

From conda-forge/numpy-feedstock#79 (comment)

One idea is to create a new flang-driver package that is just the flang executable and have it depend on clangdev (similar to how we split libflang out).

cc @isuruf

[isuruf]

Does flang.exe depend on other libraries in clangdev package?

[ghost]

Yes. So the run requirements for flang-driver would be clangdev. The run requirements for flang would be flang-driver.

Edited to resolve bootstrapping issue.

[h-vetinari]

Not sure where to put this best, but since I see this issue: we should probably create a new feedstock for flang starting with LLVM 14 (code generation will not be complete, but should help shake out issues)?