Start reifying error descriptions (Re-do of D23672 supporting VS2013)

Summary:
@kcc: I know you've accepted the other revision, but since this is a
non-trivial change, I'm updating it to show why D24029 would help.

This commit sets up the infrastructure to use reified error
descriptions, and moves ReportStackOverflow to the new system.

After we convert all the errors, we'll be able to simplify ScopedInErrorReport
and remove the older debugging mechanism which had some errors partly reified
in some way. We'll be able to maintain the external API.

ScopedInErrorReport will be able to track one of the reified errors at a time.
The purpose of this is so we have its destructor actually print the error and
possibly interface with the debugger (will depend on the platform, of course).

Reviewers: kcc, samsonov, timurrrr

Subscribers: kcc, llvm-commits, kubabrecka

Differential Revision: https://reviews.llvm.org/D24030

llvm-svn: 280111