Replace label_text ctor with "borrow" and "take"

libcpp's label_text class wraps a text buffer, along with a flag to
determine if it "owns" the buffer.

The existing ctor exposed this directly, but I found it difficult
to remember the sense of flag, so this patch hides the ctor, in
favor of static member functions "borrow" and "take", to make
the effect on ownership explicit in the name.

gcc/c-family/ChangeLog:
	* c-format.c (range_label_for_format_type_mismatch::get_text):
	Replace label_text ctor called with true with label_text::take.

gcc/c/ChangeLog:
	* c-objc-common.c (range_label_for_type_mismatch::get_text):
	Replace label_text ctor calls.

gcc/cp/ChangeLog:
	* error.c (range_label_for_type_mismatch::get_text): Replace
	label_text ctor calls with label_text::borrow.

gcc/ChangeLog:
	* gcc-rich-location.c
	(maybe_range_label_for_tree_type_mismatch::get_text): Replace
	label_text ctor call with label_text::borrow.
	* gcc-rich-location.h (text_range_label::get_text): Replace
	label_text ctor called with false with label_text::borrow.

libcpp/ChangeLog:
	* include/line-map.h (label_text::label_text): Make private.
	(label_text::borrow): New.
	(label_text::take): New.
	(label_text::take_or_copy): New.

From-SVN: r279153

libcpp/ChangeLog

@@ -1,3 +1,10 @@
+2019-12-09  David Malcolm  <dmalcolm@redhat.com>
+
+	* include/line-map.h (label_text::label_text): Make private.
+	(label_text::borrow): New.
+	(label_text::take): New.
+	(label_text::take_or_copy): New.
+
 2019-12-09  Lewis Hyatt  <lhyatt@gmail.com>
 
 	PR preprocessor/49973

libcpp/include/line-map.h

@@ -1764,18 +1764,41 @@ public:
   : m_buffer (NULL), m_caller_owned (false)
   {}
 
-  label_text (char *buffer, bool caller_owned)
-  : m_buffer (buffer), m_caller_owned (caller_owned)
-  {}
-
   void maybe_free ()
   {
     if (m_caller_owned)
       free (m_buffer);
   }
 
+  /* Create a label_text instance that borrows BUFFER from a
+     longer-lived owner.  */
+  static label_text borrow (const char *buffer)
+  {
+    return label_text (const_cast <char *> (buffer), false);
+  }
+
+  /* Create a label_text instance that takes ownership of BUFFER.  */
+  static label_text take (char *buffer)
+  {
+    return label_text (buffer, true);
+  }
+
+  /* Take ownership of the buffer, copying if necessary.  */
+  char *take_or_copy ()
+  {
+    if (m_caller_owned)
+      return m_buffer;
+    else
+      return xstrdup (m_buffer);
+  }
+
   char *m_buffer;
   bool m_caller_owned;
+
+private:
+  label_text (char *buffer, bool owned)
+  : m_buffer (buffer), m_caller_owned (owned)
+  {}
 };
 
 /* Abstract base class for labelling a range within a rich_location