[gn build] Make 'compiler-rt' depend on include dir

[thakis]

Summary

That way, the headers in llvm/utils/gn/secondary/compiler-rt/include
are copied when running ninja compiler-rt. (Previously, they were
only copied when running check-hwasan or when building the
compiler-rt/include target.)

(Since they should be copied only once, depend on the target in the
host toolchain. I think default_toolchain should work just as well,
it just needs to be a single fixed toolchain. check-hwasan depends
through host_toolchain, so let's use that here too.)

Prevents errors like

testing/fuzzed_data_provider.h:8:10: fatal error: 'fuzzer/FuzzedDataProvider.h' file not found

when building with locally-built clang. (For now, you still have to
explicitly build the 'compiler-rt' target. Maybe we should make the
clang target depend on that in the GN build?)

llvm/utils/gn/secondary/compiler-rt/BUILD.gn

 }
 if (android_ndk_path != "") {
   supported_toolchains += [
     "//llvm/utils/gn/build/toolchain:stage2_android_aarch64",
     "//llvm/utils/gn/build/toolchain:stage2_android_arm",
   ]
 }
 group("compiler-rt") {
-  deps = []
+  deps = [
+    "//compiler-rt/include($host_toolchain)",
+  ]
   foreach(toolchain, supported_toolchains) {
     deps += [ "//compiler-rt/lib($toolchain)" ]
   }
 
   # FIXME: Do this only if a gn arg compiler_rt_enable_ios is set?
   # That would match the cmake build.
   if (host_os == "mac" && have_ios_sdks) {
     if (llvm_build_AArch64) {
       deps += [ "//compiler-rt/lib/builtins(//llvm/utils/gn/build/toolchain:stage2_ios_aarch64)" ]
     }
     if (llvm_build_X86) {
       deps += [ "//compiler-rt/lib/builtins(//llvm/utils/gn/build/toolchain:stage2_iossim_x64)" ]
     }
   }
 }