This will allow using tools like Include-What-You-Use and clangd
IncludeCleaner. The tools will correctly identify the public headers
responsible for importing symbols in the testing code.
This is a backport of 100f6fbf5f
The only untouched file from that patch is
googletest/include/gtest/gtest_prod.h because the change is unrelated.
Reviewed By: sammccall
Differential Revision: https://reviews.llvm.org/D119320