[Support][test] Unconditionally use setenv macro when compiling on Windows

This test currently fails to compile when using a MinGW toolchain as setenv is not defined. This function is a POSIX function Windows does not implement.

This patch enables the setenv macro used in the unit test for all of Windows, making the test compile and run successfully.

Differential Revision: https://reviews.llvm.org/D98271

llvm/unittests/Support/ThreadPool.cpp

@@ -240,7 +240,7 @@ extern const char *TestMainArgv0;
 // Just a reachable symbol to ease resolving of the executable's path.
 static cl::opt<std::string> ThreadPoolTestStringArg1("thread-pool-string-arg1");
 
-#ifdef _MSC_VER
+#ifdef _WIN32
 #define setenv(name, var, ignore) _putenv_s(name, var)
 #endif