Remove the fixit for the diagnostics regarding capturing autoreleasing variables in a block

The fixit is actively harmful, as it encourages developers to ignore the
warning and to write unsafe code.
It is almost impossible to write safe code while capturing autoreleasing
variables in the block, as in order to check that the block is never
called in the autoreleasing pool the developer has to check the
transitive closure of all potential callers of the block.

Differential Revision: https://reviews.llvm.org/D46778

llvm-svn: 332288