[llvm] Add IWYU pragmas to GTest

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 https://github.com/google/googletest/commit/100f6fbf5f81a82d163c1e29735e8a2936eacd4f

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