Make UsingShadowDecls redeclarable. This fixes some visibility problems with
modules.

With this fixed, I no longer see any test regressions in the libc++ test suite
when enabling a single-module module.map for libc++ (other than issues with my
system headers).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193219 91177308-0d34-0410-b5e6-96231b3b80d8