Revert "Revert "C++: Implement compilation_build_mode""

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -46,6 +46,22 @@ compilation_args(
     string arg : string ref
 );
 
+/**
+ * Optionally, record the build mode for each compilation.
+ */
+compilation_build_mode(
+    unique int id : @compilation ref,
+    int mode : int ref
+);
+
+/*
+case @compilation_build_mode.mode of
+  0 = @build_mode_none
+| 1 = @build_mode_manual
+| 2 = @build_mode_auto
+;
+*/
+
 /**
  * The source files that are compiled by a compiler invocation.
  * If `id` is for the compiler invocation