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This reverts commit 20b5b8ec.
Reason for revert: Breaks existing applications, such as ANGLE
(angleproject.org), requires adding an extra prefix that needs to be
typed for no reason (when testing from command line) and increases the
logs' sizes by a non-trivial amount due to the very large number of
tests ANGLE runs.

Original commit message:

    Add a compile time check to ensure that the parameters to TEST_P and
    INSTANTIATE_TEST_SUITE_P are not empty. Some compilers already fail
    in that case and, even where it works, it's likely to result in
    technically invalid code by virtue of creating reserved identifiers:
    https://en.cppreference.com/w/cpp/language/identifiers

First, every project is perfectly capable of adding a prefix if they
want to support such a compiler. This change penalizes every
project.

Second, using a prefix such as `_p` also results in reserved
identifiers, so this change is not really solving the problem.  For that
matter, instead of generating `gtest_##prefix##...`, you can generate
`gtest_x##prefix##...` to correctly fix the issue, including when empty
prefixes are used.