Restore "[WPD/LowerTypeTests] Delay lowering/removal of type tests until after ICP"

Restore "[WPD/LowerTypeTests] Delay lowering/removal of type tests until after ICP"

This restores commit 80d0a137a5aba6998fadb764f1e11cb901aae233, and the
follow on fix in 873c0d0786dcf22f4af39f65df824917f70f2170, with a new
fix for test failures after a 2-stage clang bootstrap, and a more robust
fix for the Chromium build failure that an earlier version partially
fixed. See also discussion on D75201.

Reviewers: evgeny777

Subscribers: mehdi_amini, Prazek, hiraditya, steven_wu, dexonsmith, arphaman, davidxl, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D73242