[Support] Don't initialize compressed buffer allocated by zlib::compress

resize() (zeroing) makes every allocated page resident. The actual size of the compressed buffer is usually much
smaller. Making every page resident is wasteful.

When linking a test binary with ~1.9GiB uncompressed debug info with LLD, this optimization decreases max RSS by ~1.5GiB.

Differential Revision: https://reviews.llvm.org/50223

llvm-svn: 338913