build: canonicalise the windows paths prior to use

In order to substitute the path into the VFS overlay, we need to escape
the paths.  Instead, opt on the CMake behaviour of
`get_filename_component` providing the canonicalised path with forward
slashes rather than backslashes.  This allows the overlay to be used on
Windows as well as Linux.

Author: compnerd

cmake/modules/SwiftWindowsSupport.cmake

@@ -61,8 +61,8 @@ endfunction()
 function(swift_windows_generate_sdk_vfs_overlay flags)
   swift_verify_windows_environment_variables()
 
-  set(VCToolsInstallDir $ENV{VCToolsInstallDir})
-  set(UniversalCRTSdkDir $ENV{UniversalCRTSdkDir})
+  get_filename_component(VCToolsInstallDir $ENV{VCToolsInstallDir} ABSOLUTE)
+  get_filename_component(UniversalCRTSdkDir $ENV{UniversalCRTSdkDir} ABSOLUTE)
   set(UCRTVersion $ENV{UCRTVersion})
 
   # TODO(compnerd) use a target to avoid re-creating this file all the time