Make the Error class constructor protected

This is forcing to use Error::success(), which is in a wide majority
of cases a lot more readable.

Differential Revision: https://reviews.llvm.org/D26481

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286561 91177308-0d34-0410-b5e6-96231b3b80d8
21 files changed