ASan: use getTypeAllocSize instead of getTypeStoreSize.

ASan: use getTypeAllocSize instead of getTypeStoreSize.

This change replaces getTypeStoreSize with getTypeAllocSize in AddressSanitizer
instrumentation for stack allocations.

One case where old behaviour produced undesired results is an optimization in
InstCombine pass (PromoteCastOfAllocation), which can replace  alloca(T) with
alloca(S), where S has the same AllocSize, but a smaller StoreSize. Another
case is memcpy(long double => long double), where ASan will poison bytes 10-15
of a stack-allocated long double (StoreSize  10, AllocSize 16,
sizeof(long double) = 16).

See http://llvm.org/bugs/show_bug.cgi?id=12047 for more context.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151887 91177308-0d34-0410-b5e6-96231b3b80d8