Export of internal Abseil changes

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09c1e7877210fe85c43631538303af801c233e89 by Gennadiy Rozental <rogeeff@google.com>:

Change CordRep::length to size_t to be compatible with ChunkIterator.

PiperOrigin-RevId: 297901255

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226d5e79fb4e20fb09d75f034624d5be770f4ece by Derek Mauro <dmauro@google.com>:

Makes absl::string_view::substr constexpr for std::string_view compatibility
Fixes #627

PiperOrigin-RevId: 297872778

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851aa24a22d0ba5552098bf7e5735c95e4a8d4f7 by Abseil Team <absl-team@google.com>:

Reformat one line.

PiperOrigin-RevId: 297839574

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4f449c462583797455375fa6f1365a6b2cfa7e0a by Benjamin Barenblat <bbaren@google.com>:

Internal change

PiperOrigin-RevId: 297677173

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2d41a250e9a8f272946bc3262463e4025d88fba3 by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 297537076

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6e7fbe1e90999a58556d41955bef44033c61876c by Gennadiy Rozental <rogeeff@google.com>:

Internal change

PiperOrigin-RevId: 297376994

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5acf79338191b31cec158b06cd96ca8dfa3e81fe by Derek Mauro <dmauro@google.com>:

Add a debug-mode bounds-check on absl::Span::operator[].

PiperOrigin-RevId: 297355826

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1c540d06a56c7e92bb07b90f16b4e00b014ef18f by CJ Johnson <johnsoncj@google.com>:

Adding new LTS to the list

PiperOrigin-RevId: 297235265

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696ce48bea6927436ff89f59b887e5869b1b0f38 by Derek Mauro <dmauro@google.com>:

Fix build on FreeBSD/powerpc (implement UnscaledCycleClock)

Merges/Fixes GitHub #616

PiperOrigin-RevId: 297188640
GitOrigin-RevId: 09c1e7877210fe85c43631538303af801c233e89
Change-Id: I5d97b16bb6378792d2fcf7d29080cca18aa7729a

absl/types/span.h

@@ -276,7 +276,7 @@ class Span {
   // Returns a reference to the i'th element of this span.
   constexpr reference operator[](size_type i) const noexcept {
     // MSVC 2015 accepts this as constexpr, but not ptr_[i]
-    return *(data() + i);
+    return ABSL_ASSERT(i < size()), *(data() + i);
   }
 
   // Span::at()

absl/types/span_test.cc

@@ -232,6 +232,11 @@ TEST(IntSpan, ElementAccess) {
 
   EXPECT_EQ(s.front(), s[0]);
   EXPECT_EQ(s.back(), s[9]);
+
+#ifndef NDEBUG
+  EXPECT_DEATH_IF_SUPPORTED(s[-1], "");
+  EXPECT_DEATH_IF_SUPPORTED(s[10], "");
+#endif
 }
 
 TEST(IntSpan, AtThrows) {