-Wunguarded-availability should support if (@available) checks in top-level

blocks and lambdas

Prior to this commit Clang emitted the old "partial availability" warning for
expressions that referred to declarations that were not yet introduced in
blocks and lambdas that were not in a function/method. This commit ensures that
top-level blocks and lambdas use the new unguarded availability checks.

rdar://31835952

llvm-svn: 301409