[Allocator] Make the ContentCache object actually carry the 8-byte
alignment constraint rather than using the allocator function's over
alignment "feature". This was the only use of the "feature" which I plan
to remove next. =] Attaching the alignment to the type seems cleaner and
more principled anyways.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206324 91177308-0d34-0410-b5e6-96231b3b80d8