[flang] Fix DYLIB builds

https://reviews.llvm.org/D123211 broke builds that set both
`LLVM_BUILD_LLVM_DYLIB` and `LLVM_LINK_LLVM_DYLIB` (see [1]). This patch
fixes that.

The build failure was caused by the fact that the LLVMPasses library,
which is an LLVM "component", was listed directly as link-time
dependency. Instead, one should use `LINK_COMPONENTS` in CMake files. I
made an identical mistake recently and then subsequently fixed it in
https://reviews.llvm.org/D121461 - please visit that revision for more
detail.

I'm merging this without a review. The change is straightforward, we
recently discussed it and I was able to confirm locally that it fixes
the build issue.

[1] https://lab.llvm.org/buildbot/#/builders/177/builds/4619