[libcxx] [test] Use _putenv instead of setenv/unsetenv on windows

Move the functions to the helper header and keep the arch specific
logic there.

Differential Revision: https://reviews.llvm.org/D89681

Author: mstorsjo

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.temp_dir_path/temp_directory_path.pass.cpp

@@ -26,11 +26,11 @@
 using namespace fs;
 
 void PutEnv(std::string var, fs::path value) {
-    assert(::setenv(var.c_str(), value.string().c_str(), /* overwrite */ 1) == 0);
+    assert(utils::setenv(var.c_str(), value.string().c_str(), /* overwrite */ 1) == 0);
 }
 
 void UnsetEnv(std::string var) {
-    assert(::unsetenv(var.c_str()) == 0);
+    assert(utils::unsetenv(var.c_str()) == 0);
 }
 
 TEST_SUITE(filesystem_temp_directory_path_test_suite)

libcxx/test/support/filesystem_test_helper.h

@@ -45,11 +45,24 @@ namespace utils {
     inline int link(const char *oldname, const char* newname) {
         return !CreateHardLinkA(newname, oldname, NULL);
     }
+    inline int setenv(const char *var, const char *val, int overwrite) {
+        (void)overwrite;
+        return ::_putenv((std::string(var) + "=" + std::string(val)).c_str());
+    }
+    inline int unsetenv(const char *var) {
+        return ::_putenv((std::string(var) + "=").c_str());
+    }
 #else
     using ::mkdir;
     using ::ftruncate;
     inline int symlink(const char* oldname, const char* newname, bool is_dir) { (void)is_dir; return ::symlink(oldname, newname); }
     using ::link;
+    inline int setenv(const char *var, const char *val, int overwrite) {
+        return ::setenv(var, val, overwrite);
+    }
+    inline int unsetenv(const char *var) {
+        return ::unsetenv(var);
+    }
 #endif
 
     inline std::string getcwd() {