Mention stream-operator dead code in DCHECK docs

This adds explicit mentions for dead-code elimination of Foo() in
DCHECK() << Foo(); to c++-dos-and-donts.md, which may look like it's
"outside" the DCHECK() itself.

Bug: None
Change-Id: Ief157f2d77c66742a15ccbac9517dd3dfd7e6dda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3018441
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#900108}

styleguide/c++/c++-dos-and-donts.md

@@ -318,6 +318,12 @@ void ExpensiveStuff() { ... }  // No problem.
 // The ExpensiveStuff() call will not happen in official release builds. No need
 // to use checks for DCHECK_IS_ON() anywhere.
 DCHECK(ExpensiveStuff());
+
+std::string ExpensiveDebugMessage() { ... }  // No problem.
+
+// Calls in stream operators are also dead code in official release builds (not
+// called with the result discarded). This is fine.
+DCHECK(...) << ExpensiveDebugMessage();
 ```
 
 This code will probably do expensive things that are not needed in official