Windows - Fix handling of utf8 command line arguments

[tonydnewell]

This relates to:

Issues:

• Handling of UTF8 command argument files regressed in v26.1 breaking Grpc.Tools #17036
• Error "Failed to open argument file" when building .NET gRPC client grpc/grpc#36518
  PRs:
• Proper fix utf8 command line arguments (#14253) #15519

The fix in #15519 has one line missing that breaks Grpc.Tools and doesn't actually fix the problem it proports to fix.

This PR fixes this.

This fix needs to also be applied to the version of protobuf that is used by gRPC.

There are various scenarios that need to be tested on Windows:

A. Non-ascii command line arguments, e.g.

protoc.exe --csharp_out=out --proto_path=E:\work\grpc\protoctest\test-Dré E:\work\grpc\protoctest\test-Dré\helloworld.proto

Failed output:

E:\work\grpc\protoctest\test-DrΘ: warning: directory does not exist.
Could not make proto path relative: E:\work\grpc\protoctest\test-DrΘ\helloworld.proto: No such file or directory

Success output:

• no errors printed
• generated .cs file in the out directory

B. Non-ascii arguments in a file containing the protoc arguments (no path to file) e.g.:

protoc.exe @test.rsp

where test.rsp contains:

--plugin=protoc-gen-grpc=E:\work\grpc\protoctest\tools\grpc_csharp_plugin.exe
--csharp_out=E:\work\grpc\protoctest\test-Dré\out
--grpc_out=E:\work\grpc\protoctest\test-Dré\out
--proto_path=E:\work\grpc\protoctest\test-Dré
helloworld.proto

Success output for both old and fixed code:

• no errors printed
• generated .cs file in the out directory

C. Non-ascii arguments in a file containing the protoc arguments (with non-ascii path to file).

(This is what Grpc.Tools uses.) e.g.

protoc.exe @E:\work\grpc\protoctest\test-Dré\test.rsp

Failed output:

Failed to open argument file: E:\work\grpc\protoctest\test-DrΘ\test.rsp

Success output:

• no errors printed
• generated .cs file in the out directory

[acozzette]

@tonydnewell Thanks for the bug fix!