[Flang] Allow registering plugin extensions with the pass builder

Pass plugins are compiled and linked dynamically by default. Setting
`LLVM_${NAME}_LINK_INTO_TOOLS` to `ON` turns the project into a
statically linked extension. Projects like Polly can be used this way by
adding `-DLLVM_POLLY_LINK_INTO_TOOLS=ON` to the `cmake` command.

The changes in this patch makes the PassBuilder in Flang aware of
statically linked pass plugins, see the documentation for more details:
https://github.com/llvm/llvm-project/blob/main/llvm/docs/WritingAnLLVMNewPMPass.rst#id21

Differential Revision: https://reviews.llvm.org/D137673

Change-Id: Id1aa501dcb4821d0ec779f375cc8e8d6b0b92fce