[Bootstrap] Fix SWIFT_EXEC so it builds with "swiftc" rather than "swift".
[Bootstrap] Fix SWIFT_EXEC so it builds with "swiftc" rather than "swift".

swiftc is a symlink to swift, but the program name is significant to
the Swift driver. If realpath ends up picking "swift", turn it back
into "swiftc". This makes SWIFT_EXEC usable with the bootstrap script.

vmanot/swift-package-manager,JGiola/swift-package-manager,sschiau/swift-package-manager,sschiau/swift-package-manager,sschiau/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,vmanot/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,JGiola/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,vmanot/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,vmanot/swift-package-manager,abertelrud/swift-package-manager,vmanot/swift-package-manager