Export of internal Abseil changes.

--
2dcf3ec79f961d2962cdad33ac4bfbcb14126dad by Abseil Team <absl-team@google.com>:

Consolidate implementations of CountLeadingZeros and add new ones for
CountTrailingZeros.
Internal APIs only.

PiperOrigin-RevId: 209961247

--
4f3ac87c6928dab26faff962bb18aade9383f184 by Jorg Brown <jorg@google.com>:

Fix absl::CUnescape not to write to const_cast<char*>(str->data()), which does the wrong thing if the string type is copy-on-write

PiperOrigin-RevId: 209957656

--
c5103067be19dc88a4c32e5306154e5e3bba4673 by Tom Manshreck <shreck@google.com>:

Update comments in time library header files.

PiperOrigin-RevId: 209829588

--
fe36f9b6150243d2ac88e2a892d84c565c18cd2f by Abseil Team <absl-team@google.com>:

Clarifying the sample use of absl::LoadTimeZone function in the documentation.

PiperOrigin-RevId: 209782218
GitOrigin-RevId: 2dcf3ec79f961d2962cdad33ac4bfbcb14126dad
Change-Id: I3e2f87a8c543599b81eada58409a9bddc0b51ab8

absl/strings/escaping.cc

@@ -304,7 +304,7 @@ bool CUnescapeInternal(absl::string_view source, bool leave_nulls_escaped,
   ptrdiff_t dest_size;
   if (!CUnescapeInternal(source,
                          leave_nulls_escaped,
-                         const_cast<char*>(dest->data()),
+                         &(*dest)[0],
                          &dest_size,
                          error)) {
     return false;