[lldb] Put Host/common headers in a module

This directory escaped the modularization effort it seems. Just adding
this to the Host module along with the other common headers, which should
make this code less likely to break under modules and speed up compilation.

lldb/include/lldb/module.modulemap

@@ -55,6 +55,11 @@ module lldb_Host {
   module Time { header "Host/Time.h" export * }
   module XML { header "Host/XML.h" export * }
 
+  module common {
+    umbrella "Host/common"
+    module * { export * }
+  }
+
   export *
 }