[asan][docs] Fix the documentation to use clang++ for the C++ example

[zaks.anna]

Summary

After Darwin has been updated not to link in stdc++ on Darwin this actually started to break.

[kcc]

LGTM

cfe/trunk/docs/AddressSanitizer.rst

.. code-block:: console
     % cat example_UseAfterFree.cc
     int main(int argc, char **argv) {
       int *array = new int[100];
       delete [] array;
       return array[argc];  // BOOM
     }
 
     # Compile and link
-    % clang -O1 -g -fsanitize=address -fno-omit-frame-pointer example_UseAfterFree.cc
+    % clang++ -O1 -g -fsanitize=address -fno-omit-frame-pointer example_UseAfterFree.cc
 
 or:
 
 .. code-block:: console
 
     # Compile
-    % clang -O1 -g -fsanitize=address -fno-omit-frame-pointer -c example_UseAfterFree.cc
+    % clang++ -O1 -g -fsanitize=address -fno-omit-frame-pointer -c example_UseAfterFree.cc
     # Link
-    % clang -g -fsanitize=address example_UseAfterFree.o
+    % clang++ -g -fsanitize=address example_UseAfterFree.o
 
 If a bug is detected, the program will print an error message to stderr and
 exit with a non-zero exit code. AddressSanitizer exits on the first detected error.
 This is by design:
 
 * This approach allows AddressSanitizer to produce faster and smaller generated code
   (both by ~5%).
 * Fixing bugs becomes unavoidable. AddressSanitizer does not produce