Fix memory corruption in CStringChecker

Summary:
"register" functions for the checker were caching the checker objects in a
static variable. This caused problems when the function is called with a
different CheckerManager.

Reviewers: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D955

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183823 91177308-0d34-0410-b5e6-96231b3b80d8