[MLIR] Fix linkage for libMLIR.so

Generally:
1) don't use target_link_libraries() and add_mlir_library() on the same target, use LINK_LIBS PUBLIC instead.
2) don't use LINK_LIBS to specify LLVM libraries.  Use LINK_COMPONENTS instead
3) no need to link against LLVMSupport.  We pull it in by default.

Differential Revision: https://reviews.llvm.org/D80076