Update module llvm20 and fix symlinks (#165)

During integration of `llvm20` in a `bzl_mod` environment, following
points needed to be fixed:
* incorrect sysroot
* no valid symlinks.

Author: martin4861

MODULE.bazel

@@ -25,15 +25,19 @@ swift_cc_toolchain_ext = use_extension("@rules_swiftnav//cc:extensions.bzl", "sw
 use_repo(
     swift_cc_toolchain_ext,
     "aarch64-darwin-llvm",
+    "aarch64-darwin-llvm20",
     "aarch64-linux-llvm",
+    "aarch64-linux-llvm20",
     "aarch64-linux-musl",
     "aarch64-sysroot",
     "arm-linux-musleabihf",
     "darwin_gcc_arm_embedded_toolchain",
     "gcc_arm_gnu_8_3_toolchain",
     "llvm_mingw_toolchain",
     "x86_64-darwin-llvm",
+    "x86_64-darwin-llvm20",
     "x86_64-linux-llvm",
+    "x86_64-linux-llvm20",
     "x86_64-linux-musl",
     "x86_64-sysroot",
     "x86_64_linux_gcc_arm_embedded_toolchain",

cc/extensions.bzl

@@ -10,12 +10,14 @@ load(
     "gcc_arm_gnu_8_3_toolchain",
     "llvm_mingw_toolchain",
     "swift_cc_toolchain",
+    "swift_cc_toolchain_llvm20",
     "x86_64_linux_musl_toolchain",
     "x86_64_sysroot",
 )
 
 def _swift_cc_toolchain_impl(_ctx):
     swift_cc_toolchain()
+    swift_cc_toolchain_llvm20()
     aarch64_sysroot()
     llvm_mingw_toolchain()
     x86_64_sysroot()

cc/toolchains/llvm20/aarch64-darwin/wrappers/clang

@@ -0,0 +1 @@
+wrapper

cc/toolchains/llvm20/aarch64-darwin/wrappers/clang

@@ -0,0 +1 @@
+wrapper

cc/toolchains/llvm20/aarch64-darwin/wrappers/clang-cpp

@@ -0,0 +1 @@
+wrapper

cc/toolchains/llvm20/aarch64-darwin/wrappers/clang-cpp

@@ -0,0 +1 @@
+wrapper