Fix documentation:

  #if defined(__has_foo("X")) && __has_foo("X")

is not a correct way to portably use __has_foo, because it is expanded to

  #if 0 && 0("X")

... which is ill-formed.

Also add a missing ')'.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186047 91177308-0d34-0410-b5e6-96231b3b80d8