[analyzer] CStringChecker: Fix argument highlighting.

[NoQ]

Summary

Sometimes it's important to do BugReport->addRange() in order to know which part of the expression is problematic. It's also useful to write this down in the warning message, but i didn't bother yet.

lib/StaticAnalyzer/Checkers/CStringChecker.cpp

void CStringChecker::emitNullArgBug(CheckerContext &C, ProgramStateRef State,
   if (ExplodedNode *N = C.generateErrorNode(State)) {
     if (!BT_Null)
       BT_Null.reset(new BuiltinBug(
           Filter.CheckNameCStringNullArg, categories::UnixAPI,
           "Null pointer argument in call to byte string function"));
 
     BuiltinBug *BT = static_cast<BuiltinBug *>(BT_Null.get());
     auto Report = llvm::make_unique<BugReport>(*BT, WarningMsg, N);
+    Report->addRange(S->getSourceRange());
     bugreporter::trackNullOrUndefValue(N, S, *Report);
     C.emitReport(std::move(Report));
   }
 }
 
 void CStringChecker::emitOutOfBoundsBug(CheckerContext &C,
                                         ProgramStateRef State, const Stmt *S,
                                         StringRef WarningMsg) const {

test/Analysis/cstring-ranges.c

+// RUN: %clang_analyze_cc1 -analyzer-checker=unix.cstring -analyzer-output=text %s 2>&1 | FileCheck %s
+
+// This test verifies argument source range highlighting.
+// Otherwise we've no idea which of the arguments is null.
+
+char *strcpy(char *, const char *);
+
+void foo() {
+  char *a = 0, *b = 0;
+  strcpy(a, b);
+}
+
+// CHECK: warning: Null pointer argument in call to string copy function
+// CHECK-NEXT: strcpy(a, b);
+// CHECK-NEXT: ^      ~