Fix several issues with compiler extensions

Fix several issues with compiler extensions

- Update documentation now that the move to monorepo has been made
- Do not tie compiler extension testing to LLVM_BUILD_EXAMPLES
- No need to specify LLVM libraries for plugins
- Add NO_MODULE option to match Polly specific requirements (i.e. building the
  module *and* linking it statically)
- Issue a warning when building the compiler extension with
  LLVM_BYE_LINK_INTO_TOOLS=ON, as it modifies the behavior of clang, which only
  makes sense for testing purpose.

Still mark llvm/test/Feature/load_extension.ll as XFAIL because of a
ManagedStatic dependency that's going to be fixed in a seperate commit.

Differential Revision: https://reviews.llvm.org/D72327