Revert "Removed c++14 option from bazel.rc"

[veblush]

Reverts #29711

It turned out that -std=c++14 is required because bazel explicitly specifies it (e.g. link) Although this change generates warnings with MSVC but it's still fine because it just ignores and it will use c++14 by default anyway.

[meteorcloudy]

This is forcing downstream projects which uses grpc to add build --cxxopt='-std=c++14' to their bazelrc file because grpc no longer builds without this flag:

./src/core/lib/gpr/useful.h:109:17: error: 'auto' not allowed in lambda parameter
      [&](const auto& x) {
                ^~~~
1 error generated.

e.g. google/flatbuffers#7535

Is it possible to fix this incompatibility?

[meteorcloudy]

Or you can move this flag to GRPC_DEFAULT_COPTS so that it also works when grpc is built as a dependency.