Avoid umbrella header crash error in remote compilation when no defau…

…lt umbrella header generated

Fix err_mmap_umbrella_clash error produced by clang that so far can only be reproduced by running a target that turn off generate_default_umbrella_header and compile in a remote execution service

rules/library.bzl

@@ -142,8 +142,9 @@ def _write_umbrella_header(
  module_name = name
 
  content = ""
-
+ extern_keyword = "extern"
  if generate_default_umbrella_header:
+ extern_keyword = "FOUNDATION_EXPORT"
  content += """\
 #ifdef __OBJC__
 # import <Foundation/Foundation.h>
@@ -165,13 +166,13 @@ def _write_umbrella_header(
  for header in public_headers:
  content += "#import \"{header}\"\n".format(header = paths.basename(header))
 
- if generate_default_umbrella_header:
- content += """
-FOUNDATION_EXPORT double {module_name}VersionNumber;
-FOUNDATION_EXPORT const unsigned char {module_name}VersionString[];
+ content += """
+{extern_keyword} double {module_name}VersionNumber;
+{extern_keyword} const unsigned char {module_name}VersionString[];
 """.format(
- module_name = module_name,
- )
+ extern_keyword = extern_keyword,
+ module_name = module_name,
+ )
 
  write_file(
  name = basename + "~",