tests: Fix module loading prefer-serialized test failure on Windows

Fixes three tests failing on Windows:

```
  Swift(windows-x86_64) :: ModuleInterface/ModuleCache/force-module-loading-mode-archs.swift
  Swift(windows-x86_64) :: ModuleInterface/ModuleCache/force-module-loading-mode-framework.swift
  Swift(windows-x86_64) :: ModuleInterface/ModuleCache/force-module-loading-mode.swift
```

These test cases remove read access to the `.swiftmodule` . The expected
behavior is that the compiler checks `fs.exists("path-to.swiftmodule")`
, determines that the file exists and chooses to use it instead of the
`.swiftinterface`. Compilation then fails because the file cannot be
read.

https://github.com/apple/swift/blob/e22cf2e993267639ad8875707cadf401eb97d98b/lib/Frontend/ModuleInterfaceLoader.cpp#L752

On Windows, we were denying `R` access, which is broader than only read
access to file contents but also includes file attributes and
permissions. This caused `fs.exists` to fail since it relies on
`fs.status`, which could not open the file with `CreateFileW`. The fix
is is to only deny `RD - read data/list directory` access.

Author: tristanlabelle

test/ModuleInterface/Inputs/make-unreadable.py

@@ -30,7 +30,7 @@
 
     for path in sys.argv[1:]:
         subprocess.call(['icacls', path, '/deny',
-                         '{}:(R)'.format(user_name)])
+                         '{}:(RD)'.format(user_name)])
 else:
     for path in sys.argv[1:]:
         subprocess.call(['chmod', 'a-r', path])