mark the multi-tool entry point as async

[tomerd]

motivation: support the package-regsitry tool that is now async

changes: mark SwiftPM multi-tool (which we use in prod) as async and await on regsitry

[tomerd]

@swift-ci smoke test

[neonichu]

Not sure why this is needed, we already have a macOS 10.15.4 and iOS 13.4 minimum deployment target?

[tomerd]

I think we ran into issues with CI if not set. happy to remove this when we bump to macOS 12?

[tomerd]

@compnerd @shahmishal windows build seems to be broken for unrelated reasons:

-- Using Experimental String Processing library for RegexBuilder (C:\Users\swift-ci\jenkins\workspace\swiftpm-PR-windows\swift-experimental-string-processing).
-- Not building swiftASTGen because swift-syntax is not available
-- Swift version: 5.9
-- Swift vendor: 
-- Swift compiler version: 
-- Embedded clang compiler version: 
-- Not building SwiftMacros because swift-syntax is not available
-- Not building ObservationMacros because swift-syntax is not available
-- Looking for include file xpc/xpc.h
-- Looking for include file xpc/xpc.h - not found
-- Skipping creating early SwiftDriver symlinks - no early SwiftDriver build found.
-- C/C++ compiler for SourceKit is set to: T:/1/./bin/clang-cl.exe
-- Looking for dispatch_block_create
-- Looking for dispatch_block_create - not found
CMake Error: File C:/Users/swift-ci/jenkins/workspace/swiftpm-PR-windows/llvm-project/llvm/stdlib/public/Cxx/std/libstdcxx.modulemap does not exist.
CMake Error at C:/Users/swift-ci/jenkins/workspace/swiftpm-PR-windows/swift/unittests/ClangImporter/CMakeLists.txt:1 (configure_file):
  configure_file Problem configuring file


CMake Warning at C:/Users/swift-ci/jenkins/workspace/swiftpm-PR-windows/swift/unittests/runtime/CMakeLists.txt:19 (message):
  Building the swift runtime using the host compiler, and not the just-built
  clang.

[tomerd]

@swift-ci smoke test macOS

[tomerd]

replaced by #6254