linkage_checker: attempt dlopen before reporting broken linkage.

When `dyld` is not able to find a library at its install name, it
attempts to find it inside its default search path.

This means that for libraries we ship that are also provided by the
system (e.g. `libc++`), `brew linkage` can fail even if the formula
still works (i.e. `brew test` succeeds).

Given this, it makes sense to attempt to `dlopen` the library first
before deciding the linkage is broken. This will be useful when we ship
Homebrew/homebrew-core#106925, which will move the install location of
`libc++`, causing `brew linkage` to fail for any formula that links with
that `libc++`.

Those formulae that link with LLVM's `libc++` will fall back to Apple's
`libc++`, which is why we see multiple `brew test`s succeed even when
`brew linkage` failed in the LLVM PR linked above.

This change helps avoid unnecessary source rebuilds for formulae that
link with LLVM's `libc++` after LLVM 15 ships.

Library/Homebrew/linkage_checker.rb

@@ -196,7 +196,14 @@ def check_dylibs(rebuild_cache:)
         rescue Errno::ENOENT
           next if harmless_broken_link?(dylib)
 
-          if (dep = dylib_to_dep(dylib))
+          basename = File.basename(dylib)
+          if dylib_found_via_dlopen(basename)
+            # `dyld` will fall back to its default search path when it cannot
+            # resolve an install name. Calling `dlopen` on the basename simulates
+            # this search procedure. If this succeeds, the formula is unlikely
+            # to be broken.
+            @system_dylibs << basename
+          elsif (dep = dylib_to_dep(dylib))
             @broken_deps[dep] |= [dylib]
           elsif MacOS.version >= :big_sur && dylib_found_via_dlopen(dylib)
             # If we cannot associate the dylib with a dependency, then it may be a system library.