Add a missing test for the __if_exists extension

MSVC does not create a new scope for the body of an __if_exists compound
statement.  Clang already gets this right today, but it was untested.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211650 91177308-0d34-0410-b5e6-96231b3b80d8