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

include/llvm/CompilerDriver/Common.td

@@ -42,9 +42,9 @@ def hidden;
 def init;
 def multi_val;
 def one_or_more;
+def optional;
 def really_hidden;
 def required;
-def zero_or_one;
 def comma_separated;
 
 // The 'case' construct.