Add a library that just contains port_platform.h (#26715)

BUILD

@@ -715,6 +715,16 @@ grpc_cc_library(
     visibility = ["@grpc:public"],
 )
 
+# A library that vends only port_platform, so that libraries that don't need
+# anything else from gpr can still be portable!
+grpc_cc_library(
+    name = "gpr_platform",
+    language = "c++",
+    public_hdrs = [
+        "include/grpc/impl/codegen/port_platform.h",
+    ],
+)
+
 grpc_cc_library(
     name = "grpc_trace",
     srcs = ["src/core/lib/debug/trace.cc"],
@@ -750,6 +760,7 @@ grpc_cc_library(
     name = "overload",
     language = "c++",
     public_hdrs = ["src/core/lib/gprpp/overload.h"],
+    deps = ["gpr_platform"],
 )
 
 grpc_cc_library(
@@ -759,7 +770,10 @@ grpc_cc_library(
     ],
     language = "c++",
     public_hdrs = ["src/core/lib/gprpp/match.h"],
-    deps = ["overload"],
+    deps = [
+        "gpr_platform",
+        "overload",
+    ],
 )
 
 grpc_cc_library(

src/core/lib/gprpp/match.h

@@ -15,7 +15,7 @@
 #ifndef GRPC_CORE_LIB_GPRPP_MATCH_H
 #define GRPC_CORE_LIB_GPRPP_MATCH_H
 
-// Portable code. port_platform.h is not required.
+#include <grpc/impl/codegen/port_platform.h>
 
 #include "absl/types/variant.h"
 #include "src/core/lib/gprpp/overload.h"

src/core/lib/gprpp/overload.h

@@ -15,7 +15,7 @@
 #ifndef GRPC_CORE_LIB_GPRPP_OVERLOAD_H
 #define GRPC_CORE_LIB_GPRPP_OVERLOAD_H
 
-// Portable code. port_platform.h is not required.
+#include <grpc/impl/codegen/port_platform.h>
 
 #include <utility>
 

tools/run_tests/sanity/check_port_platform.py

@@ -44,8 +44,6 @@ def check_port_platform_inclusion(directory_root):
             with open(path) as f:
                 all_lines_in_file = f.readlines()
                 for index, l in enumerate(all_lines_in_file):
-                    if l == '// Portable code. port_platform.h is not required.\n':
-                        break
                     if '#include' in l:
                         if l not in [
                                 '#include <grpc/support/port_platform.h>\n',