[libFuzzer][MSVC] Make attribute-use compatible with MSVC

Summary:
Replace attributes with macros that use equivalent declspecs
for MSVC.

Reviewers: vitalybuka

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D56512

llvm-svn: 351456
GitOrigin-RevId: b795c31dffc5b689ac15640c8a7f67d7a293bc95
7 files changed