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.