Simplify compression API by decompressing into a SmallVector rather than a MemoryBuffer

This avoids an extra copy during decompression and avoids the use of
MemoryBuffer which is a weirdly esoteric device that includes unrelated
concepts like "file name" (its rather generic name is a bit misleading).

Similar refactoring of zlib::compress coming up.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205676 91177308-0d34-0410-b5e6-96231b3b80d8