Mark generated buildflag headers with IWYU always_keep pragma. (#23823)

https://chromium-review.googlesource.com/c/chromium/src/+/5560319

patches/build-write_buildflag_header.py.patch

@@ -0,0 +1,14 @@
+diff --git a/build/write_buildflag_header.py b/build/write_buildflag_header.py
+index a8b68f9e82cd408c30fba82d10b1cb47a253382a..6cc80ea04c03939765f655846a6eb63061a80b9c 100755
+--- a/build/write_buildflag_header.py
++++ b/build/write_buildflag_header.py
+@@ -86,6 +86,9 @@ def WriteHeader(options):
+     output_file.write('\n#ifndef %s\n' % options.header_guard)
+     output_file.write('#define %s\n\n' % options.header_guard)
+     output_file.write('#include "build/buildflag.h" // IWYU pragma: export\n\n')
++    # Clangd does not detect BUILDFLAG_INTERNAL_* indirect usage, so mark the
++    # header as "always_keep" to avoid "unused include" warning.
++    output_file.write('// IWYU pragma: always_keep\n\n')
+
+     for pair in options.flags:
+       output_file.write('#define BUILDFLAG_INTERNAL_%s() (%s)\n' % pair)