C++: Remove uniqueness constraint from uuid

Different class definitions can have the same uuid. This happens for
example when using `#import <msxml6.dll>` there will be several C++
classes generated in `msxml6.tlh` which will share uuids with
`extern "C"` struct declarations in the system header `msxml.h`.

Notice that as far as the standard cpp QL library and queries go, we
expose `getUuid()` on `UserType` and we never try to invert it, so we
only rely on uniqueness of the `id` column in the `usertype_uuid` table,
not the `uuid` column.

Closes github/codeql-c-team#893

cpp/downgrades/e9a518baf14f4322ac243578a8e1391386ff030f/upgrade.properties

@@ -0,0 +1,2 @@
+description: Remove uniqueness constraint from the uuid property
+compatibility: full

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -703,7 +703,7 @@ usertype_final(unique int id: @usertype ref);
 
 usertype_uuid(
     unique int id: @usertype ref,
-    unique string uuid: string ref
+    string uuid: string ref
 );
 
 mangled_name(

cpp/ql/lib/upgrades/bb0f279f2acd793105a347d589b5afc8715d94c4/upgrade.properties

@@ -0,0 +1,2 @@
+description: Remove uniqueness constraint from the uuid property
+compatibility: full