add externalInclude information to compilation_context

Hi bazel team

I use an aspect to run clangtidy on the codebase. It is based on https://github.com/erenon/bazel_clang_tidy.

To check my codebase with `-Werror`, I use the "new" feature `external_include_path` requested by #12009, introduced by commit 08936ae.

Now my clangtidy rules did not work anymore of course. As you can see in https://github.com/erenon/bazel_clang_tidy/blob/master/clang_tidy/clang_tidy.bzl, all the different include paths are being passed to the clang-tidy executable. I now also needed to pass the new external_includes to `clang-tidy`.

This PR adds `external_includes` to the `compilation_context` object. I looked at the style of the other `includes` and hope it is in line with your expectations.

Closes #18094.

PiperOrigin-RevId: 548070112
Change-Id: I212420d2f888c3af088c4afbf8202c848d19722e

Author: adrianimboden

src/main/java/com/google/devtools/build/lib/rules/cpp/CcCompilationContext.java

@@ -228,6 +228,17 @@ public Depset getStarlarkIncludeDirs() {
                 .collect(ImmutableList.toImmutableList())));
   }
 
+  @Override
+  public Depset getStarlarkExternalIncludeDirs() {
+    return Depset.of(
+        String.class,
+        NestedSetBuilder.wrap(
+            Order.STABLE_ORDER,
+            getExternalIncludeDirs().stream()
+                .map(PathFragment::getSafePathString)
+                .collect(ImmutableList.toImmutableList())));
+  }
+
   @Override
   public Depset getStarlarkQuoteIncludeDirs() {
     return Depset.of(

src/main/java/com/google/devtools/build/lib/starlarkbuildapi/cpp/CcCompilationContextApi.java

@@ -84,6 +84,14 @@ public interface CcCompilationContextApi<FileT extends FileApi> extends Starlark
       structField = true)
   Depset getStarlarkIncludeDirs();
 
+  @StarlarkMethod(
+      name = "external_includes",
+      doc =
+          "Returns the set of search paths (as strings) for external header files referenced by"
+              + " angle bracket. Usually passed with -isystem.",
+      structField = true)
+  Depset getStarlarkExternalIncludeDirs();
+
   @StarlarkMethod(
       name = "quote_includes",
       doc =