Fix linking issue in cpp codegen

Declare the root symbol only once and have the other modules depending
on it emit an `extern` declaration.

Fixes #9013

tests/cpp/t9013.nim

@@ -0,0 +1,9 @@
+discard """
+  targets: "cpp"
+  cmd: "nim $target --debugger:native $options $file"
+"""
+
+# The --debugger switch is needed in order to enable the defined(nimTypeNames)
+# code path in hti.nim
+import typeinfo
+var tt: Any