Avoid name shadowing in test. Patch from STL@microsoft.com

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@272618 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/language.support/support.exception/except.nested/rethrow_if_nested.pass.cpp

@@ -76,9 +76,9 @@ int main()
                     std::rethrow_if_nested(a);
                     assert(false);
                 }
-                catch (const B& b)
+                catch (const B& b2)
                 {
-                    assert(b == B(5));
+                    assert(b2 == B(5));
                 }
             }
         }