[analyzer] Allow registering custom statically-linked analyzer checkers

[analyzer] Allow registering custom statically-linked analyzer checkers

Summary:
Add an extension point to allow registration of statically-linked Clang Static
Analyzer checkers that are not a part of the Clang tree. This extension point
employs the mechanism used when checkers are registered from dynamically loaded
plugins.

Reviewers: george.karpenkov, NoQ, xazax.hun, dcoughlin

Reviewed By: george.karpenkov

Subscribers: mgorny, mikhail.ramalho, rnkovacs, xazax.hun, szepet, a.sidorin, cfe-commits

Differential Revision: https://reviews.llvm.org/D45718

llvm-svn: 335740