[analyzer] introduce getSVal(Stmt *) helper on ExplodedNode, make sure the helper...

[analyzer] introduce getSVal(Stmt *) helper on ExplodedNode, make sure the helper is used consistently

In most cases using
`N->getState()->getSVal(E, N->getLocationContext())`
is ugly, verbose, and also opens up more surface area for bugs if an
inconsistent location context is used.

This patch introduces a helper on an exploded node, and ensures
consistent usage of either `ExplodedNode::getSVal` or
`CheckContext::getSVal` across the codebase.
As a result, a large number of redundant lines is removed.

Differential Revision: https://reviews.llvm.org/D42155

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@322753 91177308-0d34-0410-b5e6-96231b3b80d8