[flang][nfc] Update the CMake dependencies for `flangFrontend`

The "FrontendActions.cpp" file (which is one of the source files for the
`flangFrontend` library) includes "Passes.h.inc" (indirectly, via
"CLOptions.inc"). This file is generated alongside other TableGen
outputs from the `FIROptTransformsPassIncGen` CMake target. This patch
adds `FIROptTransformsPassIncGen` to the list of build dependencies for
`flangFrontend`.

The lack of this dependency might cause non-determinstic build failures,
e.g. https://lab.llvm.org/buildbot/#/builders/160/builds/6210.

Differential Revision: https://reviews.llvm.org/D121218