Mark static member functions as static in CodeViewDebug

Summary:
To improve CodeView quality for static member functions, we need to make the
static explicit.  In addition to a small change in LLVM's CodeViewDebug to
return the appropriate MethodKind, this requires a small change in Clang to
note the staticness in the debug info metadata.

Subscribers: aprantl, hiraditya

Differential Revision: https://reviews.llvm.org/D37715

llvm-svn: 313192