Expose clang-format, git-clang-format, and libclang

This makes it possible to use sandboxed versions of those tools in
scripts.

Signed-off-by: Austin Schuh <austin.linux@gmail.com>

toolchain/BUILD.llvm_repo

@@ -150,3 +150,18 @@ filegroup(
     name = "clang-tidy",
     srcs = ["bin/clang-tidy"],
 )
+
+filegroup(
+    name = "clang-format",
+    srcs = ["bin/clang-format"],
+)
+
+filegroup(
+    name = "git-clang-format",
+    srcs = ["bin/git-clang-format"],
+)
+
+filegroup(
+    name = "libclang",
+    srcs = ["lib/libclang.so"],
+)