[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

llvm-svn: 322753
diff --git a/clang/lib/StaticAnalyzer/Checkers/LocalizationChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/LocalizationChecker.cpp
index 655ce33..ed0ee9f 100644
--- a/clang/lib/StaticAnalyzer/Checkers/LocalizationChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/LocalizationChecker.cpp
@@ -1017,8 +1017,7 @@
   if (!LiteralExpr)
     return nullptr;
 
-  ProgramStateRef State = Succ->getState();
-  SVal LiteralSVal = State->getSVal(LiteralExpr, Succ->getLocationContext());
+  SVal LiteralSVal = Succ->getSVal(LiteralExpr);
   if (LiteralSVal.getAsRegion() != NonLocalizedString)
     return nullptr;