Add basic_string::__resize_default_init (from P1072)

This patch adds an implementation of __resize_default_init as
described in P1072R2. Additionally, it uses it in filesystem to
demonstrate its intended utility.

Once P1072 lands, or if it changes it's interface, I will adjust
the internal libc++ implementation to match.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@347589 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/strings/basic.string/string.modifiers/resize_default_initialized.pass.cpp

@@ -0,0 +1,63 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+
+// __resize_default_init(size_type)
+
+#include <string>
+#include <cassert>
+
+#include "test_macros.h"
+
+void write_c_str(char *buf, int size) {
+  for (int i=0; i < size; ++i) {
+    buf[i] = 'a';
+  }
+  buf[size] = '\0';
+}
+
+void test_buffer_usage()
+{
+  {
+    unsigned buff_size = 125;
+    unsigned used_size = buff_size - 16;
+    std::string s;
+    s.__resize_default_init(buff_size);
+    write_c_str(&s[0], used_size);
+    assert(s.size() == buff_size);
+    assert(strlen(s.data()) == used_size);
+    s.__resize_default_init(used_size);
+    assert(s.size() == used_size);
+    assert(s.data()[used_size] == '\0');
+    for (unsigned i=0; i < used_size; ++i) {
+      assert(s[i] == 'a');
+    }
+  }
+}
+
+void test_basic() {
+  {
+    std::string s;
+    s.__resize_default_init(3);
+    assert(s.size() == 3);
+    assert(s.data()[3] == '\0');
+    for (int i=0; i < 3; ++i)
+      s[i] = 'a' + i;
+    s.__resize_default_init(1);
+    assert(s[0] == 'a');
+    assert(s.data()[1] == '\0');
+    assert(s.size() == 1);
+  }
+}
+
+int main() {
+  test_basic();
+  test_buffer_usage();
+}