Use trailing return types inside macros

[purpleKarrot]

While #2442 is about clang-tidy warnings in googletest's own header files (which can be suppressed by simply not checking them), there are also clang-tidy warnings reported in client source code. Example:

TEST(Test, Case) {}

This causes:

test.cpp:7:1: warning: use a trailing return type for this function [modernize-use-trailing-return-type]
TEST(Test, Case) {
^
include/gtest/gtest.h:2353:42: note: expanded from macro 'TEST'
#define TEST(test_suite_name, test_name) GTEST_TEST(test_suite_name, test_name)
                                         ^
include/gtest/gtest.h:2347:3: note: expanded from macro 'GTEST_TEST'
  GTEST_TEST_(test_suite_name, test_name, ::testing::Test, \
  ^
include/gtest/internal/gtest-internal.h:1537:5: note: expanded from macro 'GTEST_TEST_'
    GTEST_DISALLOW_COPY_AND_ASSIGN_(GTEST_TEST_CLASS_NAME_(test_suite_name,   \
    ^
include/gtest/internal/gtest-port.h:699:9: note: expanded from macro 'GTEST_DISALLOW_COPY_AND_ASSIGN_'
  type& operator=(type const&) = delete
        ^

This can easily be fixed by using trailing return types for all assignment operators inside macros (there are three of them).

[derekmauro]

This is a style choice rather than a bug, and we prefer not to change the style.