Distinguish `__block` variables that are captured by escaping blocks


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.

This reapplies r341754, which was reverted in r341757 because it broke a
couple of bots. r341754 was calling markEscapingByrefs after the call to
PopFunctionScopeInfo, which caused the popped function scope to be
cleared out when the following code was compiled, for example:

$ cat test.m
struct A {

id data[10];


void foo() {

__block A v;
^{ (void)v; };


This commit calls markEscapingByrefs before calling PopFunctionScopeInfo
to prevent that from happening.


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