[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/ChrootChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/ChrootChecker.cpp
index 9e9939a..d9739373 100644
--- a/clang/lib/StaticAnalyzer/Checkers/ChrootChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/ChrootChecker.cpp
@@ -105,7 +105,7 @@
 
   // After chdir("/"), enter the jail, set the enum value JAIL_ENTERED.
   const Expr *ArgExpr = CE->getArg(0);
-  SVal ArgVal = state->getSVal(ArgExpr, C.getLocationContext());
+  SVal ArgVal = C.getSVal(ArgExpr);
 
   if (const MemRegion *R = ArgVal.getAsRegion()) {
     R = R->StripCasts();