[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/NullabilityChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/NullabilityChecker.cpp
index fa9a317..b348453 100644
--- a/clang/lib/StaticAnalyzer/Checkers/NullabilityChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/NullabilityChecker.cpp
@@ -560,8 +560,7 @@
   if (State->get<InvariantViolated>())
     return;
 
-  auto RetSVal =
-      State->getSVal(S, C.getLocationContext()).getAs<DefinedOrUnknownSVal>();
+  auto RetSVal = C.getSVal(S).getAs<DefinedOrUnknownSVal>();
   if (!RetSVal)
     return;
 
@@ -977,8 +976,7 @@
   if (DestNullability == Nullability::Unspecified)
     return;
 
-  auto RegionSVal =
-      State->getSVal(CE, C.getLocationContext()).getAs<DefinedOrUnknownSVal>();
+  auto RegionSVal = C.getSVal(CE).getAs<DefinedOrUnknownSVal>();
   const MemRegion *Region = getTrackRegion(*RegionSVal);
   if (!Region)
     return;