[ADT] Move allocate_buffer to MemAlloc.h and out of line

There's an ABI breakage here if LLVM is compiled in C++14 without
aligned allocation and a user tries to use the result with aligned
allocation. If DenseMap or unique_function is used across that ABI
boundary it will break (PR45413). Moving it out of line is a bit of
a band-aid and LLVM doesn't really give ABI guarantees at this level,
but given the number of complaints I've received over this it still
seems worth fixing.