[modules] Fix Decl's Used invariant.

The Decl::isUsed has a value for every decl. In non-module builds it is very
difficult (but possible) to break this invariant but when we walk up the redecl
chain we find the neccessary information.

When deserializing the decls from a module it is much more difficult to update
correctly this invariant. The patch centralizes the information whether a decl
is used in the canonical decl marking the entire entity as being used.

Fixes https://llvm.org/bugs/show_bug.cgi?id=27401

Patch by Cristina Cristescu and me.

Thanks to Richard Smith who helped to debug and understand the issue!

Reviewed by Richard Smith.

llvm-svn: 267691