(bug 36839) Use mb_check_encoding() if available.

(bug 36839) Use mb_check_encoding() if available.

This is not a real fix for the cause of the bug (which is a
pcre.recursion_limit that is far too low), but I do wonder
about the efficiency of using a regexp to test for valid
UTF-8 encoding. After all the regexp has to be compiled first
into a state machine.

Patch set 2: Php unit test for Language.checkTitleEncoding
Patch set 3: benchmark
Patch set 4: add benchmark for non-capturing subgroup in regexp, and
since that's faster than a capturing subgroup, use it in
checkTitleEncoding() in the regexp branch.
Patch set 5: use Tim's suggestion (once-only pattern) in the regexp
branch. Also add to benchmark.

Change-Id: I551f096921d4c9c57cbcb091b80ab5970ca86a9b