[analyzer] When memcpy'ing into a fixed-size array, do not invalidate entire region.

Change the analyzer's modeling of memcpy to be more precise when copying into fixed-size
array fields. With this change, instead of invalidating the entire containing region the
analyzer now invalidates only offsets for the array itself when it can show that the
memcpy stays within the bounds of the array.

This addresses false positive memory leak warnings of the kind reported by
krzysztof in https://llvm.org/bugs/show_bug.cgi?id=22954

(This is the second attempt, now with assertion failures resolved.)

A patch by Pierre Gousseau!

Differential Revision: http://reviews.llvm.org/D12571

llvm-svn: 248516