Fix _MSC_VER check

Use "#if defined(_MSC_VER)" instead of "#if _MSC_VER" to be consistent
with other usages in googletest and to work with the "-Wundef" warning.

PiperOrigin-RevId: 508087630
Change-Id: I29c16fd2fa51a9dfecd55e10362a020318318956

googletest/test/gtest_unittest.cc

@@ -446,7 +446,7 @@ class FormatEpochTimeInMillisAsIso8601Test : public Test {
     // tzset() distinguishes between the TZ variable being present and empty
     // and not being present, so we have to consider the case of time_zone
     // being NULL.
-#if _MSC_VER || GTEST_OS_WINDOWS_MINGW
+#if defined(_MSC_VER) || GTEST_OS_WINDOWS_MINGW
     // ...Unless it's MSVC, whose standard library's _putenv doesn't
     // distinguish between an empty and a missing variable.
     const std::string env_var =