[lld] check cache before real_path in loadDylib

In llvm#137649 symlink resolution was added when loading dylibs. This
introduced a performance regression when linking with a large
number of inputs with LC_LOAD_DYLIB commands due to the syscall
overhead of realpath.

Refactor the change to be closer to the original:
  - first check if the given path is in the cache
  - if not, resolve it and check again
  - update cache entries of both paths to point to the same dylib

This mitigates the regression as we do not incur the realpath cost
for every loadDylib call, only once per unique path.

Author: rmaz

lld/MachO/DriverUtils.cpp

@@ -229,19 +229,31 @@ static DenseMap<CachedHashStringRef, DylibFile *> loadedDylibs;
 
 DylibFile *macho::loadDylib(MemoryBufferRef mbref, DylibFile *umbrella,
                             bool isBundleLoader, bool explicitlyLinked) {
-  // Frameworks can be found from different symlink paths, so resolve
-  // symlinks before looking up in the dylib cache.
-  SmallString<128> realPath;
-  std::error_code err = fs::real_path(mbref.getBufferIdentifier(), realPath);
-  CachedHashStringRef path(!err ? uniqueSaver().save(StringRef(realPath))
-                                : mbref.getBufferIdentifier());
+  CachedHashStringRef path(mbref.getBufferIdentifier());
   DylibFile *&file = loadedDylibs[path];
   if (file) {
     if (explicitlyLinked)
       file->setExplicitlyLinked();
     return file;
   }
 
+  // Frameworks can be found from different symlink paths, so resolve
+  // symlinks and look up in the dylib cache.
+  DylibFile *&realfile = file;
+  SmallString<128> realPath;
+  std::error_code err = fs::real_path(mbref.getBufferIdentifier(), realPath);
+  if (!err) {
+    CachedHashStringRef resolvedPath(uniqueSaver().save(StringRef(realPath)));
+    realfile = loadedDylibs[resolvedPath];
+    if (realfile) {
+      if (explicitlyLinked)
+        realfile->setExplicitlyLinked();
+
+      file = realfile;
+      return realfile;
+    }
+  }
+
   DylibFile *newFile;
   file_magic magic = identify_magic(mbref.getBuffer());
   if (magic == file_magic::tapi_file) {
@@ -253,6 +265,7 @@ DylibFile *macho::loadDylib(MemoryBufferRef mbref, DylibFile *umbrella,
     }
     file =
         make<DylibFile>(**result, umbrella, isBundleLoader, explicitlyLinked);
+    realfile = file;
 
     // parseReexports() can recursively call loadDylib(). That's fine since
     // we wrote the DylibFile we just loaded to the loadDylib cache via the
@@ -268,6 +281,7 @@ DylibFile *macho::loadDylib(MemoryBufferRef mbref, DylibFile *umbrella,
            magic == file_magic::macho_executable ||
            magic == file_magic::macho_bundle);
     file = make<DylibFile>(mbref, umbrella, isBundleLoader, explicitlyLinked);
+    realfile = file;
 
     // parseLoadCommands() can also recursively call loadDylib(). See comment
     // in previous block for why this means we must copy `file` here.