Update fastpath to catch opening tags.

Summary:
If you have something like:

  $foo = <div>Her's</div>;

The fastpath won't pick up the XHP because it thinks it's inside of a string, and it only looks for closing tags, not opening tags. I added an expression to detect opening tags as well, which in practice won't have many false positives.

Test Plan:
New unit test. Also ran xhpize on FB sources and no new fastpath false positives were introduced.

Reviewed By: epriestley

tests/apos-fastpath.phpt

@@ -0,0 +1,9 @@
+--TEST--
+Apostrophe Fastpath
+--FILE--
+<?php
+$foo = xhp_preprocess_code('<?php
+$foo = <div>Her\'s</div>;foo(\'\');');
+echo isset($foo['new_code']);
+--EXPECT--
+1

xhp/fastpath.re

@@ -98,6 +98,7 @@ bool xhp_fastpath(const char* yy, const size_t len, const xhp_flags_t &flags) {
     <PHP> '::' { continue; }
     <PHP> '</'|
           '/>'|
+          '<'[a-zA-Z0-9:_\-]+'>'|
           ':'[a-zA-Z0-9]|
           ')'WHITESPACE*'['|
           '&#' {