Reland "[clang][Fuchsia] Support availability attr on Fuchsia"

This reland commit 1131b1eb, which
adds support to __attribute__((availability)) annotation for Fuchsia
platform. This patch also adds '-ffuchsia-api-level' to allow specify
Fuchsia API level from the command line.

Differential Revision: https://reviews.llvm.org/D108592