Fix a hypothetical problem for targets with StackGrowsUp and a non-zero

Fix a hypothetical problem for targets with StackGrowsUp and a non-zero
LocalAreaOffset.  (We don't have any of those right now.)
PEI::calculateFrameObjectOffsets includes the absolute value of the
LocalAreaOffset in the cumulative offset value used to calculate the
stack frame size.  It then adds the raw value of the LocalAreaOffset
to the stack size.  For a StackGrowsDown target, that raw value is negative
and has the effect of cancelling out the absolute value that was added
earlier, but that obviously won't work for a StackGrowsUp target.  Change
to subtract the absolute value of the LocalAreaOffset.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82693 91177308-0d34-0410-b5e6-96231b3b80d8