[Sanitizer] Remove brittle cache variable and slightly simplify blacklisting code.
Now CodeGenFunction is responsible for looking at sanitizer blacklist
(in CodeGenFunction::StartFunction) and turning off instrumentation,
if necessary.

No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212501 91177308-0d34-0410-b5e6-96231b3b80d8