Rewrite flycheck-parse-xml-region (use xml-parse-region if libxml fails)

Closes GH-1298, GH-1330.

flycheck.el

@@ -5521,19 +5521,25 @@ function `buffer-file-name'."
 (defun flycheck-parse-xml-region (beg end)
   "Parse the xml region between BEG and END.
 
-Wrapper around `xml-parse-region' which transforms the return
-value of this function into one compatible to
+Try parsing with libxml first; if that fails, revert to
+`xml-parse-region'.  Failures can be caused by slightly incorrect
+XML (see URL `https://github.com/flycheck/flycheck/issues/1298'),
+or on Windows by a missing libxml DLL with a libxml-enabled Emacs
+build (see URL `https://github.com/flycheck/flycheck/issues/1330').
+
+The output of `xml-parse-region' is made compatible with that of
 `libxml-parse-xml-region' by simply returning the first element
-from the node list."
-  (car (xml-parse-region beg end)))
+of the node list."
+  ;; FIXME use `libxml-available-p' when it gets implemented.
+  (or (and (fboundp 'libxml-parse-xml-region)
+           (libxml-parse-xml-region beg end))
+      (ignore-errors (car (xml-parse-region beg end)))))
 
-(defvar flycheck-xml-parser
-  (if (fboundp 'libxml-parse-xml-region)
-      'libxml-parse-xml-region 'flycheck-parse-xml-region)
-  "Parse an xml string from a region.
+(defvar flycheck-xml-parser 'flycheck-parse-xml-region
+  "Function used to parse an xml string from a region.
 
-Use libxml if Emacs is built with libxml support.  Otherwise fall
-back to `xml-parse-region', via `flycheck-parse-xml-region'.")
+The default uses libxml if available, and falls back to
+`xml-parse-region' otherwise.")
 
 (defun flycheck-parse-xml-string (xml)
   "Parse an XML string.