Validate the generated C++ code in llvmc tests.

Validate the generated C++ code in llvmc tests.

Checks that the code generated by 'tblgen --emit-llvmc' can be actually
compiled. Also fixes two bugs found in this way:

- forward_transformed_value didn't work with non-list arguments
- cl::ZeroOrOne is now called cl::Optional

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@91404 91177308-0d34-0410-b5e6-96231b3b80d8