[msan] Annotate zlib functions for MemorySanitizer.

Mark destination buffer in zlib::compress and zlib::decompress as fully
initialized.

When building LLVM with system zlib and MemorySanitizer instrumentation,
MSan does not observe memory writes in zlib code and erroneously considers
zlib output buffers as uninitialized, resulting in false use-of-uninitialized
memory reports. This change helps MSan understand the state of that memory
and prevents such reports.