Cherry-pick of df719723: Simplify template exporting macros

[stanley-cheung]

This is for grpc/grpc#23011 and #6659

Apparently this commit: df71972 has never been merged properly internally, and that got inadvertently merged back here in #7344.

So this PR attempts to re-apply this commit df71972

Apparently the commit ID df71972 == 479ec14, and we are cherry-picking this one particular commit in #6869.

[stanley-cheung]

With this fix, it got past my original error, but now I got a new one

In file included from third_party/protobuf/src/google/protobuf/stubs/port.h:43:0,
                 from third_party/protobuf/src/google/protobuf/stubs/common.h:46,
                 from third_party/protobuf/src/google/protobuf/stubs/casts.h:34,
                 from third_party/protobuf/src/google/protobuf/message.h:118,
                 from include/grpcpp/impl/codegen/config_protobuf.h:30,
                 from ./src/compiler/config_protobuf.h:22,
                 from ./src/compiler/config.h:22,
                 from src/compiler/csharp_generator.cc:24:
third_party/protobuf/src/google/protobuf/port_def.inc:436:9: error: '#pragma' is not allowed here
 #pragma GCC diagnostic push
         ^~~
third_party/protobuf/src/google/protobuf/port_def.inc:437:52: error: expected declaration before end of line
 #pragma GCC diagnostic ignored "-Winvalid-offsetof"
                                                    ^
make: *** [Makefile:2949: /var/local/git/grpc/objs/dbg/src/compiler/csharp_generator.o] Error 1

There seems to be still more we need to do in this PR.

[stanley-cheung]

Here's how to reproduce the error above:

The environment is Alpine 3.5 + CONFIG=dbg

git clone https://github.com/stanley-cheung/grpc ./grpc-protobuf-pr-test
cd grpc-protobuf-pr-test/
git checkout proto3-presence-protoc-plugin
git submodule update --init --recursive
docker build -t grpc-alpine -f ./tools/dockerfile/test/cxx_alpine_x64/Dockerfile .  

[Edit:] Updating my base image from Alpine 3.5 to 3.9 seem to have fixed my problem. So this PR is good enough as is to fix my original problem.

[egorpugin]

I see errors after this change (3.12.1->3.12.2).
I use custom build system, maybe I need to adjust something, but build spec was not changed for many versions already.

When executing: [org.sw.demo.google.protobuf.protoc_lib-3.12.2]/src/google/protobuf/compiler/objectivec/objectivec_helpers.cc
D:\dev\swst\pkg\20\2f\b237\src\sdir\src\google/protobuf/parse_context.h(749): error C2491: 'google::protobuf::internal::PackedEnumParser': definition of dllimport function not allowed
D:\dev\swst\pkg\20\2f\b237\src\sdir\src\google/protobuf/parse_context.h(765): error C2491: 'google::protobuf::internal::PackedEnumParserArg': definition of dllimport function not allowed