Allow constructing PassRepeated from a PassManager

[vlstill]

... and add explicit to the constructors that could be accidentally invoced by implicit casts otherwise

[grg]

LGTM.

Is the explicit to prevent accidental casts on the repeats argument? Or are there cases where an accidental cast could occur on the first argument? (Can something be cast to a PassManager?)

[vlstill]

Is the explicit to prevent accidental casts on the repeats argument? Or are there cases where an accidental cast could occur on the first argument? (Can something be cast to a PassManager?)

It is the first argument. Any non-explicit constructor that can be called with a single argument can be used for explicit cast. So the new constructor would introduce cast from PassManager to PassRepeated, while the original had introduced cast from initializer list to PassRepeated. There are probably more in the compiler and it would be best to use clang-tidy to check and fix these.