[libcxx] [test] Rename b to b2 in order to avoid shadowing between nested catch blocks.

[STL_MSFT]

Summary

Rename b to b2 in order to avoid shadowing between nested catch blocks.

Fixes MSVC warning C6246 "Local declaration of 'b' hides declaration of the same name in outer scope."

[EricWF]

r272618.

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

int main()
             }
             catch (const A& a)
             {
                 try
                 {
                     std::rethrow_if_nested(a);
                     assert(false);
                 }
-                catch (const B& b)
+                catch (const B& b2)
                 {
-                    assert(b == B(5));
+                    assert(b2 == B(5));
                 }
             }
         }
     }
     {
         try
         {
             std::rethrow_if_nested(C());