[K/N] Update LLVM distributions to 11.1.0

This commit is important for several reasons:
1. Finally, LLVM update from 8.0.0 to 11.1.0
At the time of writing, LLVM 12.0.0 already came out, but we need to be
compatible with LLVM from Xcode 12.5, so that's why we stick to a bit
older version of LLVM.
2. These LLVM distributions are built with tools/llvm_builder/package.py
We finally managed to introduce an explicit process of building our LLVM
distributions, so we will be able to updated them more frequently and
adapt to our needs.
3. Native Windows LLVM instead of MinGW
Last but not least, we now use native Windows LLVM distribution. While
it might be harder to use (no more posix everywhere), simpler build
process (building msys2 is not that easy) and no need for MinGW-specific
patches makes such distribution way more convenient.

kotlin-native/tools/llvm_builder/package.py

@@ -234,7 +234,7 @@ def clone_llvm_repository(repo, branch, llvm_repo_destination):
     Downloads a single commit from the given repository.
     """
     if host_is_darwin():
-        default_repo, default_branch = "https://github.com/apple/llvm-project", "apple/stable/20200108"
+        default_repo, default_branch = "https://github.com/apple/llvm-project", "apple/stable/20200714"
     else:
         default_repo, default_branch = "https://github.com/llvm/llvm-project", "release/11.x"
     repo = default_repo if repo is None else repo
@@ -245,12 +245,8 @@ def clone_llvm_repository(repo, branch, llvm_repo_destination):
 
 
 def default_num_stages():
-    if host_is_darwin():
-        # Bootstrap build is not working for https://github.com/apple/llvm-project + apple/stable/20200108.
-        # So use single phase for now.
-        return 1
-    else:
-        return 2
+    # Perform bootstrap build
+    return 2
 
 
 def build_parser() -> argparse.ArgumentParser: