compilerplugins: move oncevar into store

As suggested by Stephan and agreed by Noel -- as its warnings are not
useful in all cases.

Change-Id: I3c6034ae9d3c74aeb089a67dc48c1f91a3ea3a21
diff --git a/compilerplugins/clang/oncevar.cxx b/compilerplugins/clang/store/oncevar.cxx
similarity index 100%
rename from compilerplugins/clang/oncevar.cxx
rename to compilerplugins/clang/store/oncevar.cxx