[gn build] Make NOSORT line actually work

[thakis]

Summary

GN wants the NOSORT line to be the first line of a comment block, not the last line.

I sent https://gn-review.googlesource.com/c/gn/+/3560 to support having it in the last line too, but since it will be a while until everyone has that change even if it's expected, use the form that works today.

[phosek]

LGTM

llvm/trunk/utils/gn/secondary/clang/lib/Headers/BUILD.gn

copy("arm_headers") {
   ]
 }
 
 copy("Headers") {
   deps = [
     ":arm_headers",
   ]
 
+  # NOSORT
   # Tell `gn format` to not reorder the sources list: Its order matches CMake,
   # and the ordering is alphabetical but ignores leading underscores.
-  # NOSORT
   sources = [
     "adxintrin.h",
     "altivec.h",
     "ammintrin.h",
     "arm_acle.h",
     "armintr.h",
     "arm64intr.h",
     "avx2intrin.h",