Fix an off-by-one error in basic_string::__grow_by, where it would incorrectly throw length_error (instead of bad_alloc) when attempting to resize the string to 'max_size()'. Add tests for resizing to max_size +/-1

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@194151 91177308-0d34-0410-b5e6-96231b3b80d8