[analyzer] Add annotation attribute to trust retain count implementation

Add support to the retain-count checker for an annotation indicating that a
function's implementation should be trusted by the retain count checker.
Functions with these attributes will not be inlined and the arguments will
be treating as escaping.

Adding this annotation avoids spurious diagnostics when the implementation of
a reference counting operation is visible but the analyzer can't reason
precisely about the ref count.

Patch by Malhar Thakkar!

Differential Revision: https://reviews.llvm.org/D34937

llvm-svn: 308416