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Distinguish `__block` variables that are captured by escaping blocks

Description

Distinguish __block variables that are captured by escaping blocks
from those that aren't.

This patch changes the way __block variables that aren't captured by
escaping blocks are handled:

  • Since non-escaping blocks on the stack never get copied to the heap (see https://reviews.llvm.org/D49303), Sema shouldn't error out when the type of a non-escaping __block variable doesn't have an accessible copy constructor.
  • IRGen doesn't have to use the specialized byref structure (see https://clang.llvm.org/docs/Block-ABI-Apple.html#id8) for a non-escaping __block variable anymore. Instead IRGen can emit the variable as a normal variable and copy the reference to the block literal. Byref copy/dispose helpers aren't needed either.

rdar://problem/39352313

Differential Revision: https://reviews.llvm.org/D51564