Enable shared linking to LLVM on non-Windows

Windows doesn't quite support dynamic linking to LLVM yet, but on other
platforms we do. In #76708, it was discovered that we dynamically link to LLVM
from the LLVM tools (e.g., rust-lld), so we need the shared LLVM library to link
against. That means that if we do not have a shared link to LLVM, and want LLVM
tools to work, we'd be shipping two copies of LLVM on all of these platforms:
one in librustc_driver and one in libLLVM.

Also introduce an error into rustbuild if we do end up configured for shared
linking on Windows.

src/bootstrap/native.rs

@@ -129,6 +129,10 @@ impl Step for Llvm {
                 Err(m) => m,
             };
 
+        if builder.config.llvm_link_shared && target.contains("windows") {
+            panic!("shared linking to LLVM is not currently supported on Windows");
+        }
+
         builder.info(&format!("Building LLVM for {}", target));
         t!(stamp.remove());
         let _time = util::timeit(&builder);

src/ci/run.sh

@@ -75,6 +75,13 @@ if [ "$DEPLOY$DEPLOY_ALT" = "1" ]; then
   RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set rust.remap-debuginfo"
   RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --debuginfo-level-std=1"
 
+  # If we're distributing binaries, we want a shared LLVM link. We're already
+  # going to link LLVM to the LLVM tools dynamically, so we need to ship a
+  # libLLVM library anyway.
+  if !isWindows; then
+    RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set llvm.link-shared=true"
+  fi
+
   if [ "$NO_LLVM_ASSERTIONS" = "1" ]; then
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --disable-llvm-assertions"
   elif [ "$DEPLOY_ALT" != "" ]; then