Export of internal Abseil changes.

--
fe689b30f9a3a614e8a577997cc340043d01c2f1 by Abseil Team <absl-team@google.com>:

Change arm32 linux backtrace to use the generic implementation.

PiperOrigin-RevId: 218386158

--
86f8678d055d32edc989e6a4d1dc49c3a15cd350 by Abseil Team <absl-team@google.com>:

Update documentation on SHARED_LOCKS_REQUIRED and EXCLUSIVE_LOCKS_REQUIRED so that users know the differences between them.

PiperOrigin-RevId: 218365545

--
22947b48ce4a1ba71ad4794f762235dac3a1df12 by Greg Falcon <gfalcon@google.com>:

The from_chars implementation incorrectly assumed `uint32_t` was `unsigned int`.

`strings_internal::BigUnsigned` had `uint32_t` and `uint64_t` constructors; when both of these types differ from `unsigned int`, `BigUnsigned(1u)` is ambiguous (neither conversion is better).

Fix this by removing the `uint32_t` constructor.  When the `uint64_t` constructor is called with a literal or type that is 32 bits or narrower, the compiler is smart enough to optimize away the two-word case, so this fix is free.

PiperOrigin-RevId: 218346935

--
7201ab430bb90ca0e30b102915d02564f61353eb by Abseil Team <absl-team@google.com>:

Fix formatting errors discovered during merge conflict.

PiperOrigin-RevId: 218229891
GitOrigin-RevId: fe689b30f9a3a614e8a577997cc340043d01c2f1
Change-Id: I5d382482ad227d48ffe57b243ce11b1eb44a1314

absl/debugging/internal/stacktrace_config.h

@@ -41,8 +41,9 @@
 #define ABSL_STACKTRACE_INL_HEADER \
     "absl/debugging/internal/stacktrace_aarch64-inl.inc"
 # elif defined(__arm__)
+// Note: When using glibc this may require -funwind-tables to function properly.
 #define ABSL_STACKTRACE_INL_HEADER \
-    "absl/debugging/internal/stacktrace_arm-inl.inc"
+  "absl/debugging/internal/stacktrace_generic-inl.inc"
 # else
 #define ABSL_STACKTRACE_INL_HEADER \
    "absl/debugging/internal/stacktrace_unimplemented-inl.inc"

absl/debugging/internal/stacktrace_generic-inl.inc

@@ -19,6 +19,7 @@
 template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
 static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
                       const void *ucp, int *min_dropped_frames) {
+  static_cast<void>(ucp);  // Unused.
   static const int kStackLength = 64;
   void * stack[kStackLength];
   int size;