[analyzer] Avoid querying this-pointers for static-methods.

[analyzer] Avoid querying this-pointers for static-methods.

Summary:
The loop-widening code processes c++ methods looking for `this` pointers.  In
the case of static methods (which do not have `this` pointers), an assertion
was triggering.   This patch avoids trying to process `this` pointers for
static methods, and thus avoids triggering the assertion .

Reviewers: dcoughlin, george.karpenkov, NoQ

Reviewed By: NoQ

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

Differential Revision: https://reviews.llvm.org/D50408

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@339201 91177308-0d34-0410-b5e6-96231b3b80d8