Put code that avoids heapifying local blocks behind a flag

This change puts the functionality in commit
c5792aa9 behind a flag that is off by
default.  The original commit is not in Apple's Clang fork (and blocks
are an Apple extension in the first place), and there is one known
issue that needs to be addressed before it can be enabled safely.

Differential Revision: https://reviews.llvm.org/D108243