.gitattributes: add symlink target types

This is a first step toward removing the fix_symlinks() hack from
tools/third_party.py.

.gitattributes

 # Use Unix line endings in all text files.
 * text=auto eol=lf
+
+# Tell git which symlinks point to files, and which ones point to directories.
+# This is revelevant for Windows only, and requires git >= 2.19.2 to work.
+/* symlink=dir
+/website/* symlink=dir
+/tools/clang symlink=dir

third_party @ 7f6923ab

-Subproject commit e94296a27cdaeb96bfd86e499162764409e503bc
+Subproject commit 7f6923ab6e7125238a69d4c0dd8b31f12cb78ce7