Switch libLLVMFrontendOpenACC to be a regular CMake library and not a "component"

This library is only used in Flang at the moment and not tested withing LLVM.
Having it as a component is breaking llvm-config:

  $ bin/llvm-config --shared-mode
  llvm-config: error: component libraries and shared library

  llvm-config: error: missing: [...]/lib/libLLVMFrontendOpenACC.a

This will reverted when unit-tests are provided for it.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D91470