Add a fix-it for -Wunguarded-availability

This patch adds a fix-it for the -Wunguarded-availability warning. This fix-it
is similar to the Swift one: it suggests that you wrap the statement in an
`if (@available)` check. The produced fixits are indented (just like the Swift
ones) to make them look nice in Xcode's fix-it preview.

rdar://31680358

Differential Revision: https://reviews.llvm.org/D32424

llvm-svn: 302253