Don't link any LLVM/MLIR library to the C API unit-test

[mehdi_amini]

Summary

The tests are intended to exercise the public C API and will link to a
specific shared library exposing only the C API, this library itself may
link to libMLIR.so.
If we link some LLVM library statically in the test themselves, we end
up with duplicated cl::opt registrations in LLVM. A possible setup if
these libraries were needed could be to link libMLIR.so directly when
available and link statically when it isn't available (in which case the
libary exposing the C API would be statically link and isolated from the
cl::opt registry, hopefully).

[stellaraccident]

Thanks, Mehdi! This helps unblock the dynamic linking work, and gives us some time to ponder a more holistic design.

mlir/test/CAPI/CMakeLists.txt

 set(LLVM_OPTIONAL_SOURCES
   ir.c
   pass.c
 )
 
 set(LLVM_LINK_COMPONENTS
-  Core
-  Support
   )
 
 add_llvm_executable(mlir-capi-ir-test
   ir.c
   )
 llvm_update_compile_flags(mlir-capi-ir-test)
 
 get_property(dialect_libs GLOBAL PROPERTY MLIR_DIALECT_LIBS)