Add custom_umbrella_header to ios_static_framework

This attribute allows users to specify an umbrella header to use for the
framework in place of generating a default one. This is useful for
frameworks that have multiple headers, including a natural umbrella
header, that has a conflicting name with the one that would be
generated.

apple/internal/ios_rules.bzl

@@ -311,6 +311,7 @@ def _ios_static_framework_impl(ctx):
         partials.binary_partial(binary_artifact = binary_artifact),
         partials.static_framework_header_modulemap_partial(
             hdrs = ctx.files.hdrs,
+            umbrella_header = ctx.file.umbrella_header,
             binary_objc_provider = binary_target[apple_common.Objc],
         ),
     ]

apple/internal/partials/static_framework_header_modulemap.bzl

@@ -115,14 +115,19 @@ def _create_umbrella_header(actions, output, headers):
     content = "\n".join(import_lines) + "\n"
     actions.write(output = output, content = content)
 
-def _static_framework_header_modulemap_partial_impl(ctx, hdrs, binary_objc_provider):
+def _static_framework_header_modulemap_partial_impl(ctx, hdrs, umbrella_header, binary_objc_provider):
     """Implementation for the static framework headers and modulemaps partial."""
     bundle_name = bundling_support.bundle_name(ctx)
 
     bundle_files = []
 
     umbrella_header_name = None
-    if hdrs:
+    if umbrella_header:
+        umbrella_header_name = umbrella_header.basename
+        bundle_files.append(
+            (processor.location.bundle, "Headers", depset(hdrs + [umbrella_header])),
+        )
+    elif hdrs:
         umbrella_header_name = "{}.h".format(bundle_name)
         umbrella_header_file = intermediates.file(ctx.actions, ctx.label.name, umbrella_header_name)
         _create_umbrella_header(
@@ -165,13 +170,14 @@ def _static_framework_header_modulemap_partial_impl(ctx, hdrs, binary_objc_provi
         bundle_files = bundle_files,
     )
 
-def static_framework_header_modulemap_partial(hdrs, binary_objc_provider):
+def static_framework_header_modulemap_partial(hdrs, umbrella_header, binary_objc_provider):
     """Constructor for the static framework headers and modulemaps partial.
 
     This partial bundles the headers and modulemaps for static frameworks.
 
     Args:
       hdrs: The list of headers to bundle.
+      umbrella_header: An umbrella header to use instead of generating one
       binary_objc_provider: The ObjC provider for the binary target.
 
     Returns:
@@ -181,5 +187,6 @@ def static_framework_header_modulemap_partial(hdrs, binary_objc_provider):
     return partial.make(
         _static_framework_header_modulemap_partial_impl,
         hdrs = hdrs,
+        umbrella_header = umbrella_header,
         binary_objc_provider = binary_objc_provider,
     )

apple/internal/rule_factory.bzl

@@ -525,6 +525,15 @@ use only extension-safe APIs.
 A list of `.h` files that will be publicly exposed by this framework. These headers should have
 framework-relative imports, and if non-empty, an umbrella header named `%{bundle_name}.h` will also
 be generated that imports all of the headers listed here.
+""",
+            ),
+            "umbrella_header": attr.label(
+                allow_single_file = [".h"],
+                doc = """
+A single .h file to use as the umbrella header for this framework. Usually, this header will have the
+same name as this target, so that clients can load the header using the #import <MyFramework/MyFramework.h>
+format. If this attribute is not specified, an umbrella header will be generated under the same name as this
+target.
 """,
             ),
             "avoid_deps": attr.label_list(