[flang] Fix frontend build with -DBUILD_SHARED_LIBS=On

Fix fronted shared library builds by eliminating dependences of
the parser on other component libraries, moving some code around that
wasn't in the right library, and making some dependences
explicit in the CMakeLists.txt files.  The lowering library
does not yet build as a shared library due to some undefined
names.

Reviewed By: tskeith

Differential Revision: https://reviews.llvm.org/D83515