Re-land new pass manager coro-split and coro-elide

This re-applies patches https://reviews.llvm.org/D71899 and
https://reviews.llvm.org/D71900, which were reverted in
https://reviews.llvm.org/rG11053a1cc61 and
https://reviews.llvm.org/rGe999aa38d16. The underlying problem that
caused two buildbots to fail with these patches is explained in
https://reviews.llvm.org/rG26f356350bd -- older compliers disagree with
the order in which the left- and right-hand side of an assignment in
LazyCallGraph ought to be evaluated, which caused an assertion in
SmallVector::operator[] to fire when the test suite was run.