Ensure LLVM is in the link path for "fulldeps" tests

This is a follow-up to #70123, which added `llvm-config --libdir` to the
`LIBRARY_PATH` for rustc tools. We need the same for "run-make-fulldeps"
and "ui-fulldeps" tests which depend on compiler libraries, implicitly
needing to link to `-lLLVM` as well.

src/bootstrap/test.rs

@@ -21,7 +21,7 @@ use crate::flags::Subcommand;
 use crate::native;
 use crate::tool::{self, SourceType, Tool};
 use crate::toolstate::ToolState;
-use crate::util::{self, dylib_path, dylib_path_var};
+use crate::util::{self, add_link_lib_path, dylib_path, dylib_path_var};
 use crate::Crate as CargoCrate;
 use crate::{envify, DocTests, GitRepo, Mode};
 
@@ -1178,6 +1178,15 @@ impl Step for Compiletest {
                 cmd.arg("--system-llvm");
             }
 
+            // Tests that use compiler libraries may inherit the `-lLLVM` link
+            // requirement, but the `-L` library path is not propagated across
+            // separate compilations. We can add LLVM's library path to the
+            // platform-specific environment variable as a workaround.
+            if !builder.config.dry_run && suite.ends_with("fulldeps") {
+                let llvm_libdir = output(Command::new(&llvm_config).arg("--libdir"));
+                add_link_lib_path(vec![llvm_libdir.trim().into()], &mut cmd);
+            }
+
             // Only pass correct values for these flags for the `run-make` suite as it
             // requires that a C++ compiler was configured which isn't always the case.
             if !builder.config.dry_run && suite == "run-make-fulldeps" {