[flang] Disable Plugins in out-of-tree builds

https://reviews.llvm.org/D106137 added support for plugins in Flang. The
CMake configuration for plugins requires some LLVM variables that are
not available in out-of-tree builds (e.g. `LLVM_SHLIB_OUTPUT_INTDIR`).
This has caused the out-of-tree BuildBot worker to start failing:
  * https://lab.llvm.org/buildbot/#/builders/175

This patch effectively disables plugins in out-of-tree builds and fixes
the configuration error. In order to support plugins in out-of-tree
builds, we would have to find a way to access the missing CMake
variables from LLVM. This could be implemented at a later time.

Differential Revision: https://reviews.llvm.org/D107973