disable array bounds overflow warning for cases where an array

disable array bounds overflow warning for cases where an array
has a single element.  This disables the warning in cases where
there is a clear bug, but this is really rare (who uses arrays
with one element?) and it also silences a large class of false
positive issues with C89 code that is using tail padding in structs.

A better version of this patch would detect when an array is in
a tail position in a struct, but at least patch fixes the huge
false positives that are hitting postgres and other code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@136724 91177308-0d34-0410-b5e6-96231b3b80d8