[analyzer] Rename trackNullOrUndefValue to trackExpressionValue

trackNullOrUndefValue is a long and confusing name,
and it does not actually reflect what the function is doing.
Give a function a new name, with a relatively clear semantics.

Also remove some dead code.

Differential Revision: https://reviews.llvm.org/D52758

llvm-svn: 345064
diff --git a/clang/lib/StaticAnalyzer/Checkers/NullabilityChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/NullabilityChecker.cpp
index 86ae9cb..ad68dc2 100644
--- a/clang/lib/StaticAnalyzer/Checkers/NullabilityChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/NullabilityChecker.cpp
@@ -174,7 +174,8 @@
       if (Error == ErrorKind::NilAssignedToNonnull ||
           Error == ErrorKind::NilPassedToNonnull ||
           Error == ErrorKind::NilReturnedToNonnull)
-        bugreporter::trackNullOrUndefValue(N, ValueExpr, *R);
+        if (const auto *Ex = dyn_cast<Expr>(ValueExpr))
+          bugreporter::trackExpressionValue(N, Ex, *R);
     }
     BR.emitReport(std::move(R));
   }