Revert "[stdlib]Enable Swift 4 mode for the overlays"

[shahmishal]

Reverts #12608

FAILED: lib/swift/macosx/x86_64/libswiftFoundation.dylib 
: && /Users/buildnode/jenkins/workspace/oss-swift-incremental-RA-osx/buildbot_incremental/llvm-macosx-x86_64/./bin/clang++  -Wno-unknown-warning-option -Werror=unguarded-availability-new -fno-stack-protector -stdlib=libc++ -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -std=c++11 -Wall -W -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wcovered-switch-default -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -fcolor-diagnostics -Werror=switch -Wdocumentation -Wimplicit-fallthrough -Wunreachable-code -Woverloaded-virtual -DOBJC_OLD_DISPATCH_PROTOTYPES=0 -fno-sanitize=all -DLLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING=1 -O3 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -dynamiclib -Wl,-headerpad_max_install_names -stdlib=libc++   -target x86_64-apple-macosx10.9 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk -arch x86_64 -F /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/../../../Developer/Library/Frameworks -mmacosx-version-min=10.9 -Wl,-sectcreate,__TEXT,__info_plist,/Users/buildnode/jenkins/workspace/oss-swift-incremental-RA-osx/buildbot_incremental/swift-macosx-x86_64/stdlib/public/SDK/Foundation/Info.plist -Wl,-application_extension  "-L/Users/buildnode/jenkins/workspace/oss-swift-incremental-RA-osx/buildbot_incremental/swift-macosx-x86_64/./lib/swift/macosx/x86_64" "-L/Users/buildnode/jenkins/workspace/oss-swift-incremental-RA-osx/buildbot_incremental/swift-macosx-x86_64/./bin/../lib/swift/macosx/x86_64" "-L/Users/buildnode/jenkins/workspace/oss-swift-incremental-RA-osx/buildbot_incremental/swift-macosx-x86_64/./bin/../lib/swift/macosx" -o lib/swift/macosx/x86_64/libswiftFoundation.dylib -install_name @rpath/libswiftFoundation.dylib stdlib/public/SDK/Foundation/macosx/x86_64/Foundation.o stdlib/public/SDK/Foundation/CMakeFiles/swiftFoundation-macosx-x86_64.dir/DataThunks.m.o stdlib/public/SDK/Foundation/CMakeFiles/swiftFoundation-macosx-x86_64.dir/CheckClass.mm.o -L/Users/buildnode/jenkins/workspace/oss-swift-incremental-RA-osx/buildbot_incremental/llvm-macosx-x86_64/./lib -framework Foundation lib/swift/macosx/x86_64/libswiftCoreGraphics.dylib lib/swift/macosx/x86_64/libswiftIOKit.dylib lib/swift/macosx/x86_64/libswiftCoreFoundation.dylib lib/swift/macosx/x86_64/libswiftCore.dylib -framework Foundation lib/swift/macosx/x86_64/libswiftDarwin.dylib -framework CoreFoundation lib/swift/macosx/x86_64/libswiftDispatch.dylib -framework IOKit lib/swift/macosx/x86_64/libswiftObjectiveC.dylib -framework CoreGraphics && :
Undefined symbols for architecture x86_64:
  "__T0So13DispatchQueueC0A0E10AttributesVN", referenced from:
      __T0s22_ContiguousArrayBufferVAByxGSi19_uninitializedCount_Si15minimumCapacitytcfCSo13DispatchQueueC0H0E10AttributesV_Tgq5Tf4nnd_n in Foundation.o
ld: symbol(s) not found for architecture x86_64
clang-5.0: error: linker command failed with exit code 1 (use -v to see invocation)

[shahmishal]

@swift-ci smoke test

[lancep]

The error message doesn't look related to my change, we should try a clean build before reverting.