[gn build] Add build files for unittests that load shared libraries

This is slightly ugly for three reasons:

- The shlib needs to go next to the binary to be found on all platforms, so the
  build files refer to target_out_dir
- The explicit -fPIC flag needed on the shared lib side, and the -rdynamic flag
  needed on the host side, on Linux 
- Plugins that refer to LLVM code and assume that the host will resolve them
  don't work on Windows -- PluginsTests won't test anything on Windows (but
  DynamicLibraryTests will, since the dll here doesn't call LLVM code)


If we get lots more of these plugin / plugin host targets it might make sense
to add a template for them. But for now, these are the last ones we need.

(We're at 6 plugin hosts, 2 of them tests, and at 6 shared libraries, 2 of them
tests as well. clang is a plugin host by default in the CMake build but not
(yet?) in the GN build.)

Differential Revision: https://reviews.llvm.org/D56330

llvm-svn: 350454

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