Reland "Fetch clang-format automatically when compiling .fp files."

This is a reland of a25422faa462e8cfb00af1a3818547abbbb9c7f9

Original change's description:
> Fetch clang-format automatically when compiling .fp files.
>
> On a freshly fetched repo, setting `skia_compile_processors = true` will
> fail to compile because clang-format is missing from the bin directory.
> This CL automatically runs fetch-clang-format for you when clang-format
> is absent.
>
> Change-Id: Ieeb359176072e92ca235316c820310333732f608
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295780
> Reviewed-by: Mike Klein <mtklein@google.com>
> Commit-Queue: John Stiles <johnstiles@google.com>

Change-Id: Ib397716771f742f192ebd6ed7ec0a9915b2400c5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295956
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 143ce5a..fb39ead 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -492,6 +492,7 @@
     args = [
       rebase_path(sksllex_path),
       rebase_path("bin/clang-format"),
+      rebase_path("bin/fetch-clang-format"),
       rebase_path("src"),
     ]
   }
@@ -558,6 +559,7 @@
     args = [
       rebase_path(skslc_path),
       rebase_path("bin/clang-format"),
+      rebase_path("bin/fetch-clang-format"),
     ]
     args += rebase_path(skia_gpu_processor_sources)
   }