[IRMemoryMap] Shrink Allocation and make it move-only (NFC)

Profiling data show that Allocation::operator= is hot (see the data
attached to the Phab review).

Reorder a few fields within Allocation to avoid implicit structure
padding and shrink the structure. This should make copies a bit cheaper.

Also, given that an Allocation contains a std::vector (by way of
DataBufferHeap), it's preferable to make it move-only instead of
permitting expensive copies. As an added benefit this allows us to have
a single Allocation constructor instead of two.

Differential Revision: https://reviews.llvm.org/D50271

llvm-svn: 339290