Reword warning about using a *static* variable within its own initialization.

The warning is still under -Wuninitialized (although technically this
is defined behavior), but under a subgroup -Wstatic-self-init.

This addresses PR 10265.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172878 91177308-0d34-0410-b5e6-96231b3b80d8