Revert "Generalize the Mangling of Constrained Existential Types"

[nate-chandler]

Provisionally reverts #59763 to see whether that might fix these build errors

/Users/ec2-user/jenkins/workspace/oss-swift_tools-RA_stdlib-RDA_long-test/swift/stdlib/public/runtime/MetadataLookup.cpp:1336:5: error: unknown type name 'BuiltType'
    BuiltType Ptr;
    ^
/Users/ec2-user/jenkins/workspace/oss-swift_tools-RA_stdlib-RDA_long-test/swift/stdlib/public/runtime/MetadataLookup.cpp:1340:25: error: unknown type name 'BuiltType'
    BigBuiltTypeIntPair(BuiltType ptr, RequirementKind i) : Ptr(ptr), Int(i) {}
                        ^
/Users/ec2-user/jenkins/workspace/oss-swift_tools-RA_stdlib-RDA_long-test/swift/stdlib/public/runtime/MetadataLookup.cpp:1342:5: error: unknown type name 'BuiltType'
    BuiltType getPointer() const { return Ptr; }
    ^
/Users/ec2-user/jenkins/workspace/oss-swift_tools-RA_stdlib-RDA_long-test/swift/stdlib/public/runtime/MetadataLookup.cpp:1375:11: error: member initializer 'RequirementBase' does not name a non-static data member or base class
        : RequirementBase(kind, first, second) {}
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/ec2-user/jenkins/workspace/oss-swift_tools-RA_stdlib-RDA_long-test/swift/stdlib/public/runtime/MetadataLookup.cpp:1378:11: error: member initializer 'RequirementBase' does not name a non-static data member or base class
        : RequirementBase(kind, first, second) {}
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/ec2-user/jenkins/workspace/oss-swift_tools-RA_stdlib-RDA_long-test/swift/stdlib/public/runtime/MetadataLookup.cpp:1336:15: warning: private field 'Ptr' is not used [-Wunused-private-field]
    BuiltType Ptr;
              ^
1 warning and 5 errors generated.

seen on the following bots:

https://ci.swift.org/job/oss-swift-incremental-RA-macos/980/
https://ci.swift.org/job/oss-swift-package-macos/652/
https://ci.swift.org/job/oss-swift-pr-test-macoss/750/
https://ci.swift.org/job/oss-swift_tools-R_stdlib-RD_test-simulator/885/
https://ci.swift.org/job/oss-swift_tools-RA_stdlib-DA_test-device-non_executable/1084/
https://ci.swift.org/job/oss-swift_tools-RA_stdlib-DA_test-simulators/535/
https://ci.swift.org/job/oss-swift_tools-RA_stdlib-RD_test-simulator/778/
https://ci.swift.org/job/oss-swift_tools-RA_stdlib-RDA_long-test/721/
https://ci.swift.org/job/oss-swift_tools-RA_stdlib-RDA_test-simulator/595/
https://ci.swift.org/job/swift-main-sourcekitd-stress-tester/130/

[nate-chandler]

@swift-ci please test

[nate-chandler]

buildbot,tools=RA,stdlib=RDA,long_test
@swift-ci Please test with preset macOS

[rjmccall]

I think #59842 might be less disruptive, but if it doesn't fix the problem, we'll need to do a full revert.

[nate-chandler]

@rjmccall Absolutely. Let's leave this running just in case we need it.