[ProfData] Detect if zlib is available

As discussed on [1], if the profile is compressed and llvm-profdata is not built with zlib support, the error message is not informative. Give a better error message if zlib is not available.

[1] http://lists.llvm.org/pipermail/llvm-dev/2017-July/115571.html

Reviewers: davidxl, dblaikie

Differential Revision: https://reviews.llvm.org/D35586

llvm-svn: 308789