XDG: Windows: resolve $LOCALAPPDATA, $TEMP (#5278)

After #4964 environment variables in the XDG "fallback" table are no
longer expanded.

Fallback to correctly expanded $LOCALAPPDATA, $TEMP. If that fails
(unlikely), fallback to hard-coded paths (e.g. ~/AppData/Local).

Closes #5255

test/functional/options/defaults_spec.lua

@@ -9,8 +9,6 @@ local eval = helpers.eval
 local eq = helpers.eq
 local neq = helpers.neq
 
-if helpers.pending_win32(pending) then return end
-
 local function init_session(...)
   local args = { helpers.nvim_prog, '-i', 'NONE', '--embed',
     '--cmd', 'set shortmess+=I background=light noswapfile noautoindent',
@@ -24,6 +22,8 @@ end
 
 describe('startup defaults', function()
   describe(':filetype', function()
+    if helpers.pending_win32(pending) then return end
+
     local function expect_filetype(expected)
       local screen = Screen.new(48, 4)
       screen:attach()
@@ -99,8 +99,37 @@ describe('startup defaults', function()
 end)
 
 describe('XDG-based defaults', function()
-  -- Need to be in separate describe() block to not run clear() twice.
+  -- Need separate describe() blocks to not run clear() twice.
   -- Do not put before_each() here for the same reasons.
+
+  describe('with empty/broken environment', function()
+    it('sets correct defaults', function()
+      clear({env={
+        XDG_CONFIG_HOME=nil,
+        XDG_DATA_HOME=nil,
+        XDG_CACHE_HOME=nil,
+        XDG_RUNTIME_DIR=nil,
+        XDG_CONFIG_DIRS=nil,
+        XDG_DATA_DIRS=nil,
+        LOCALAPPDATA=nil,
+        HOMEPATH=nil,
+        HOMEDRIVE=nil,
+        HOME=nil,
+        TEMP=nil,
+        VIMRUNTIME=nil,
+        USER=nil,
+      }})
+
+      eq('.', meths.get_option('backupdir'))
+      eq('.', meths.get_option('viewdir'))
+      eq('.', meths.get_option('directory'))
+      eq('.', meths.get_option('undodir'))
+    end)
+  end)
+
+  -- TODO(jkeyes): tests below fail on win32 because of path separator.
+  if helpers.pending_win32(pending) then return end
+
   describe('with too long XDG variables', function()
     before_each(function()
       clear({env={