Making the gpr_tmpfile's win32 version a bit more Windows-y.

Also adding windows helpers to convert to and from TCHAR strings.
diff --git a/build.json b/build.json
index 170b148..579ede3 100644
--- a/build.json
+++ b/build.json
@@ -231,6 +231,7 @@
         "src/core/support/file.h",
         "src/core/support/murmur_hash.h",
         "src/core/support/string.h",
+        "src/core/support/string_win32.h",
         "src/core/support/thd_internal.h"
       ],
       "src": [