Warn when a static variable is referenced in a non-static inline function.

Warn when a static variable is referenced in a non-static inline function.

This is explicitly forbidden in C99 6.7.4p3. This is /not/ forbidden in C++,
probably because by default file-scope const/constexpr variables have internal
linkage, while functions have external linkage. There's also the issue of
anonymous namespaces to consider. Nevertheless, there should probably be a
similar warning, since the semantics of inlining a function that references
a variable with internal linkage do not seem well-defined.

<rdar://problem/11577619>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158531 91177308-0d34-0410-b5e6-96231b3b80d8