Reduce our reliance on COPTS variable for compiler flags

It's somewhat tedious to explicitly set this option on all of our C++ targets,
so I think ideally we should rely primarily on bazelrc files for setting
compiler flags. I tried to completely remove `COPTS`, but unfortunately that
did not work out--we have so many `-Wsign-compare` warnings that I think we
need to keep suppressing them for now or else we will get a lot of complaints.

However, I was able to get to the point where `-Wno-sign-compare` is the only
flag we need in `COPTS` for non-Windows builds. I explicitly set `-DHAVE_ZLIB`
on just the two targets that need it, and removed `-Wno-nonnull` since we are
already compliant with that warning. I moved `-Woverloaded-virtual` to our
bazelrc files so that CI will enforce that we remain compliant with that.

PiperOrigin-RevId: 684863987

build_defs/cpp_opts.bzl

@@ -17,10 +17,7 @@ COPTS = select({
         "/wd4996",  # The compiler encountered a deprecated declaration.
     ],
     "//conditions:default": [
-        "-DHAVE_ZLIB",
-        "-Woverloaded-virtual",
         "-Wno-sign-compare",
-        "-Wno-nonnull",
     ],
 })
 

ci/Linux.bazelrc

@@ -1,4 +1,5 @@
 import common.bazelrc
 
 build --cxxopt=-std=c++14 --host_cxxopt=-std=c++14
+build --cxxopt="-Woverloaded-virtual"
 build --copt="-Werror" --copt="-Wno-sign-compare" --copt="-Wno-sign-conversion" --copt="-Wno-error=sign-conversion" --copt="-Wno-deprecated-declarations"

ci/macOS.bazelrc

@@ -1,6 +1,7 @@
 import common.bazelrc
 
 build --cxxopt=-std=c++14 --host_cxxopt=-std=c++14
+build --cxxopt="-Woverloaded-virtual"
 build --copt="-Werror" --copt="-Wno-sign-compare" --copt="-Wno-sign-conversion" --copt="-Wno-error=sign-conversion" --copt="-Wno-deprecated-declarations"
 common --repo_env=BAZEL_NO_APPLE_CPP_TOOLCHAIN=1
-common --xcode_version_config=@com_google_protobuf//.github:host_xcodes
+common --xcode_version_config=@com_google_protobuf//.github:host_xcodes

src/google/protobuf/io/BUILD.bazel

@@ -150,7 +150,10 @@ cc_library(
     name = "gzip_stream",
     srcs = ["gzip_stream.cc"],
     hdrs = ["gzip_stream.h"],
-    copts = COPTS,
+    copts = COPTS + select({
+        "//build_defs:config_msvc": [],
+        "//conditions:default": ["-DHAVE_ZLIB"],
+    }),
     strip_include_prefix = "/src",
     deps = [
         ":io",
@@ -192,7 +195,10 @@ cc_test(
         "tokenizer_unittest.cc",
         "zero_copy_stream_unittest.cc",
     ],
-    copts = COPTS,
+    copts = COPTS + select({
+        "//build_defs:config_msvc": [],
+        "//conditions:default": ["-DHAVE_ZLIB"],
+    }),
     deps = [
         ":gzip_stream",
         ":io",