Changes imported from Abseil "staging" branch:

- 5e874e644191fbf99f5636d6303de2b28b23392c Adds a absl::apply function, similar to c++17's std::apply. by Abseil Team <absl-team@google.com>
  - 16373c438d16a09725dace03ab7ba0f7c2337279 Add debugging_internal::StackTraceWorksForTest by Abseil Team <absl-team@google.com>
  - a623257aaaff8a5fba3377f34f92f319a104e444 Update absl::CondVar documentation in response to GitHub ... by Derek Mauro <dmauro@google.com>
  - 87d58a25bc4ecd46165dd1c417121c86cbc07be0 Add assert against uint128 negative bit shift undefined b... by Alex Strelnikov <strel@google.com>
  - af155c0d2a3556b56a9bcd6f9ee7416277185df8 Fix comment typos. by Abseil Team <absl-team@google.com>
  - 1824ae832eb75d447dea730b5968d952897e135a Rollback of: Add debugging_internal::StackTraceWorksForTest by Abseil Team <absl-team@google.com>
  - 97318f087ce63dd5acf1e0d3d697cd90a7d6ebfd Add debugging_internal::StackTraceWorksForTest by Abseil Team <absl-team@google.com>
  - 9dd1d17dca17f0ded3bda336b7521fd57d08a5cc Move log_severity.h out of internal. by Abseil Team <absl-team@google.com>
  - 2212bb56b1a8365d2303ff0983441298d08444e5 Internal change. by Alex Strelnikov <strel@google.com>

GitOrigin-RevId: 5e874e644191fbf99f5636d6303de2b28b23392c
Change-Id: Ic270393ac1f15866afb64617d28269cd829030f6

absl/debugging/internal/stacktrace_aarch64-inl.inc

@@ -178,4 +178,12 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
   return n;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return true;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_AARCH64_INL_H_

absl/debugging/internal/stacktrace_arm-inl.inc

@@ -112,4 +112,12 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
   return n;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return false;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_ARM_INL_H_

absl/debugging/internal/stacktrace_generic-inl.inc

@@ -48,4 +48,12 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
   return result_count;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return true;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_GENERIC_INL_H_

absl/debugging/internal/stacktrace_powerpc-inl.inc

@@ -232,4 +232,12 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
   return n;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return true;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_POWERPC_INL_H_

absl/debugging/internal/stacktrace_unimplemented-inl.inc

@@ -11,4 +11,12 @@ static int UnwindImpl(void** /* result */, int* /* sizes */,
   return 0;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return false;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_UNIMPLEMENTED_INL_H_

absl/debugging/internal/stacktrace_win32-inl.inc

@@ -72,4 +72,12 @@ static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
   return n;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return false;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_WIN32_INL_H_

absl/debugging/internal/stacktrace_x86-inl.inc

@@ -326,4 +326,12 @@ static int UnwindImpl(void **result, int *sizes, int max_depth, int skip_count,
   return n;
 }
 
+namespace absl {
+namespace debugging_internal {
+bool StackTraceWorksForTest() {
+  return true;
+}
+}  // namespace debugging_internal
+}  // namespace absl
+
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_X86_INL_INC_

absl/debugging/stacktrace.h

@@ -155,6 +155,13 @@ extern int DefaultStackUnwinder(void** pcs, int* sizes, int max_depth,
                                 int skip_count, const void* uc,
                                 int* min_dropped_frames);
 
+namespace debugging_internal {
+// Returns true for platforms which are expected to have functioning stack trace
+// implementations. Intended to be used for tests which want to exclude
+// verification of logic known to be broken because stack traces are not
+// working.
+extern bool StackTraceWorksForTest();
+}  // namespace debugging_internal
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_STACKTRACE_H_