I can reproduce this, even with command-line gdb:
(gdb) p *str.d
yields
$3 = {<QArrayData> = {ref = {atomic = {_q_value = 135154040 } } ,
size = 134905058 , alloc = 1 , capacityReserved = 0 , offset = -1073745292 , ...}
Note the unrealistic 'size' and 'alloc' values.
So either gdb does not read debug information properly (unlikely) or the compiler produced wrong debug info, possible due to applying some optimization without adjusting debug information
(more likely, happened in the past, see e.g. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44731)
There is really nothing Qt Creator can do in this case.
Note also that the problem does not appear when passing the parameter as const &.