Add a new warning -Wmissing-variable-declarations, to warn about variables

defined without a previous declaration.  This is similar to
-Wmissing-prototypes, but for variables instead of functions.

Patch by Ed Schouten.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@166498 91177308-0d34-0410-b5e6-96231b3b80d8