[ConstraintSystem] Allow getPotentialBindings to find supertypes of o…

[rudkx]

…ptionals.

Improve enumerateDirectSupertypes so that for T? it will return U? if
U is a supertype of T. This is another form of direct supertype.

This is making up for a deficiency in the completeness of our Type
join implementation, which should be able to directly compute a join
of disparate types that share some common supertype, but sometimes
fails to in cases involving protocol compositions.

Fixes rdar://problem/45490737

(cherry picked from commit 85237aa)

[rudkx]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - ce6879a

[rudkx]

Linux failure is in configuring:

10:50:15 CMake Error at CMakeLists.txt:58 (find_package):
10:50:15   Could not find a package configuration file provided by "Swift" with any of
10:50:15   the following names:
10:50:15 
10:50:15     SwiftConfig.cmake
10:50:15     swift-config.cmake
10:50:15 
10:50:15   Add the installation prefix of "Swift" to CMAKE_PREFIX_PATH or set
10:50:15   "Swift_DIR" to a directory containing one of the above files.  If "Swift"
10:50:15   provides a separate development package or SDK, be sure it has been
10:50:15   installed.