[libcxx] Add "flag" default arg: basic_regex ptr_size_flag ctor

Summary:
The synopsis in C++11 subclause 28.8 [re.regex] has:
```
basic_regex(const charT* p, size_t len,
            flag_type f = regex_constants::ECMAScript);
```

The default argument is added to libc++ by this change.

Reviewers: mclow.lists, rsmith, hubert.reinterpretcast

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D22702

Reapplies r277966.
Patch by Jason Liu!

llvm-svn: 277968

libcxx/include/regex

@@ -147,7 +147,7 @@ public:
     // construct/copy/destroy:
     basic_regex();
     explicit basic_regex(const charT* p, flag_type f = regex_constants::ECMAScript);
-    basic_regex(const charT* p, size_t len, flag_type f);
+    basic_regex(const charT* p, size_t len, flag_type f = regex_constants::ECMAScript);
     basic_regex(const basic_regex&);
     basic_regex(basic_regex&&) noexcept;
     template <class ST, class SA>
@@ -2519,7 +2519,7 @@ public:
           __end_(0)
         {__parse(__p, __p + __traits_.length(__p));}
     _LIBCPP_INLINE_VISIBILITY
-    basic_regex(const value_type* __p, size_t __len, flag_type __f)
+    basic_regex(const value_type* __p, size_t __len, flag_type __f = regex_constants::ECMAScript)
         : __flags_(__f), __marked_count_(0), __loop_count_(0), __open_count_(0),
           __end_(0)
         {__parse(__p, __p + __len);}

libcxx/test/std/re/re.regex/re.regex.construct/ptr_size.pass.cpp

@@ -0,0 +1,39 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <regex>
+
+// template <class charT, class traits = regex_traits<charT>> class basic_regex;
+
+// basic_regex(const charT* p, size_t len);
+
+#include <regex>
+#include <cassert>
+
+template <class CharT>
+void
+test(const CharT* p, std::size_t len, unsigned mc)
+{
+    std::basic_regex<CharT> r(p, len);
+    assert(r.flags() == std::regex_constants::ECMAScript);
+    assert(r.mark_count() == mc);
+}
+
+int main()
+{
+    test("\\(a\\)", 5, 0);
+    test("\\(a[bc]\\)", 9, 0);
+    test("\\(a\\([bc]\\)\\)", 13, 0);
+    test("(a([bc]))", 9, 2);
+
+    test("(\0)(b)(c)(d)", 12, 4);
+    test("(\0)(b)(c)(d)", 9, 3);
+    test("(\0)(b)(c)(d)", 3, 1);
+    test("(\0)(b)(c)(d)", 0, 0);
+}