Handle new LLVM soname

LLVM now includes the minor version in the soname, and also changed
the names of library files. libLLVM-18.so is now a symlink to
libLLVM.so.18.1.

We need to make two changes to support this: First, we need to
run the installed llvm-config binary, rather than the one from
the build directory. This is because the symlink does not exist
in the build directory, but llvm-config requires it. This looks
like an LLVM bug to me, but it's probably a good idea to use the
installed version anyway.

Second, when installing LLVM into the libdir, we need to follow
the symlink from libLLVM-18.so to libLLVM.so.18.1, as this is what
will actually get loaded at runtime.

src/bootstrap/src/core/build_steps/dist.rs

@@ -2026,7 +2026,13 @@ fn install_llvm_file(builder: &Builder<'_>, source: &Path, destination: &Path) {
         return;
     }
 
-    builder.install(source, destination, 0o644);
+    if source.is_symlink() {
+        // Follow symlinks. E.g. if we're linking against libLLVM-18.so, then what gets loaded
+        // at runtime is libLLVM.so.18.1.
+        builder.install(&t!(fs::canonicalize(source)), destination, 0o644);
+    } else {
+        builder.install(&source, destination, 0o644);
+    }
 }
 
 /// Maybe add LLVM object files to the given destination lib-dir. Allows either static or dynamic linking.

src/bootstrap/src/core/build_steps/llvm.rs

@@ -98,9 +98,6 @@ pub fn prebuilt_llvm_config(
     let out_dir = builder.llvm_out(target);
 
     let mut llvm_config_ret_dir = builder.llvm_out(builder.config.build);
-    if (!builder.config.build.is_msvc() || builder.ninja()) && !builder.config.llvm_from_ci {
-        llvm_config_ret_dir.push("build");
-    }
     llvm_config_ret_dir.push("bin");
     let build_llvm_config = llvm_config_ret_dir.join(exe("llvm-config", builder.config.build));
     let llvm_cmake_dir = out_dir.join("lib/cmake/llvm");