Fix foward declaration duplication

BUILD

CMakeLists.txt

Makefile

build.yaml

gRPC-Core.podspec

grpc.gemspec

include/grpc/impl/codegen/exec_ctx_fwd.h

include/grpc/impl/codegen/slice.h

package.xml

src/core/lib/iomgr/closure.h

test/core/surface/public_headers_must_be_c89.c

tools/doxygen/Doxyfile.c++

tools/doxygen/Doxyfile.c++.internal

tools/doxygen/Doxyfile.core

tools/doxygen/Doxyfile.core.internal

tools/run_tests/sources_and_headers.json

tools/run_tests/tests.json

vsprojects/vcxproj/gpr/gpr.vcxproj

vsprojects/vcxproj/gpr/gpr.vcxproj.filters

vsprojects/vcxproj/grpc++/grpc++.vcxproj