MFC r312213,r313713:
r312213:
Turn COMPILER_VERSION/COMPILER_TYPE make check into a compile-time check
of the clang version
This works around breakage on ^/stable/10 when running installworld from
a ^/stable/10 host where the test wouldn't be compiled on the first
go-around and would be missing when make installworld is run.
PR: 208703
r313713:
Handle clang 4.x+ with the compile-time exception added in r312213
It also fails the assertions noted in bug 208703