C++: Update dbscheme

Author: calumgrant

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -48,11 +48,9 @@ compilation_args(
 
 /**
  * Optionally, record the build mode for each compilation.
- * The build mode should be the same for all compilations.
  */
-#keyset[id, mode]
 compilation_build_mode(
-    int id : @compilation ref,
+    unique int id : @compilation ref,
     int mode : int ref
 );
 
@@ -62,7 +60,7 @@ case @compilation_build_mode.mode of
 | 1 = @build_mode_manual
 | 2 = @build_mode_auto
 ;
-/*
+*/
 
 /**
  * The source files that are compiled by a compiler invocation.