Add missing PATH_MAX for GNU Hurd support

[sylvestre.ledru]

Summary

If you have a better place to put them, don't hesitate to let me know :)

Patch by Svante Signell & myself

[rsmith]

lib/Basic/FileManager.cpp

514–519

This doesn't appear to be necessary: the identifier PATH_MAX does not appear later in this file.

lib/Frontend/ModuleDependencyCollector.cpp

102–106

I think this is generally the wrong way to address this problem on Hurd:

https://www.gnu.org/software/hurd/hurd/porting/guidelines.html#PATH_MAX_tt_MAX_PATH_tt_MAXPATHL

If PATH_MAX isn't defined, we should instead pass NULL to realpath (and deallocate the result): passing a fixed-size buffer is wrong, because we cannot assume that the real path will fit in 4096 bytes.

(If we neither have PATH_MAX defined nor a realpath that can accept a null pointer as the destination buffer, then realpath is unusable on that platform and there's nothing we can do there but return a false result to the caller of real_path.)

lib/Basic/FileManager.cpp

void FileManager::invalidateCache(const FileEntry *Entry) {
   SeenFileEntries.erase(Entry->getName());
 
   // FileEntry invalidation should not block future optimizations in the file
   // caches. Possible alternatives are cache truncation (invalidate last N) or
   // invalidation of the whole cache.
   UniqueRealFiles.erase(Entry->getUniqueID());
 }
 
+// For GNU Hurd
+#if defined(__GNU__) && !defined(PATH_MAX)
+# define PATH_MAX 4096
+#endif
+
+
 void FileManager::GetUniqueIDMapping(
                    SmallVectorImpl<const FileEntry *> &UIDToFiles) const {
   UIDToFiles.clear();
   UIDToFiles.resize(NextFileUID);
 
   // Map file entries
   for (llvm::StringMap<FileEntry*, llvm::BumpPtrAllocator>::const_iterator
          FE = SeenFileEntries.begin(), FEEnd = SeenFileEntries.end();

lib/Frontend/ModuleDependencyCollector.cpp

if (!UmbrellaDir.equals(UmbreallDirFromHeader)) {
       if (FileMgr->getFile(AltHeaderFilename))
         moduleMapAddHeader(AltHeaderFilename);
     }
   }
 };
 
 }
 
+// For GNU Hurd
+#if defined(__GNU__) && !defined(PATH_MAX)
+# define PATH_MAX 4096
+#endif
+
 // TODO: move this to Support/Path.h and check for HAVE_REALPATH?
 static bool real_path(StringRef SrcPath, SmallVectorImpl<char> &RealPath) {
 #ifdef LLVM_ON_UNIX
   char CanonicalPath[PATH_MAX];
 
   // TODO: emit a warning in case this fails...?
   if (!realpath(SrcPath.str().c_str(), CanonicalPath))
     return false;