Add a way to enable '-opt=foo' forwarding.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@96916 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CompilerDriver/Common.td

@@ -46,6 +46,7 @@ def optional;
 def really_hidden;
 def required;
 def comma_separated;
+def forward_not_split;
 
 // The 'case' construct.
 def case;