[analyzer] Do not invalidate the `this` pointer.

Summary:
`this` pointer is not an l-value, although we have modeled `CXXThisRegion` for `this` pointer, we can only bind it once, which is when we start to inline method. And this patch fixes https://bugs.llvm.org/show_bug.cgi?id=35506.

In addition, I didn't find any other cases other than loop-widen that could invalidate `this` pointer.

Reviewers: NoQ, george.karpenkov, a.sidorin, seaneveson, szepet

Reviewed By: NoQ

Subscribers: xazax.hun, rnkovacs, cfe-commits, MTC

Differential Revision: https://reviews.llvm.org/D45491

llvm-svn: 330095