Export of internal Abseil changes

--
c99f979ad34f155fbeeea69b88bdc7458d89a21c by Derek Mauro <dmauro@google.com>:

Remove a floating point division by zero test.

This isn't testing behavior related to the library, and MSVC warns
about it in opt mode.

PiperOrigin-RevId: 285220804

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68b015491f0dbf1ab547994673281abd1f34cd4b by Gennadiy Rozental <rogeeff@google.com>:

This CL introduces following changes to the class FlagImpl:
* We eliminate the CommandLineFlagLocks struct. Instead callback guard and callback function are combined into a single CallbackData struct, while primary data lock is stored separately.
* CallbackData member of class FlagImpl is initially set to be nullptr and is only allocated and initialized when a flag's callback is being set. For most flags we do not pay for the extra space and extra absl::Mutex now.
* Primary data guard is stored in data_guard_ data member. This is a properly aligned character buffer of necessary size. During initialization of the flag we construct absl::Mutex in this space using placement new call.
* We now avoid extra value copy after successful attempt to parse value out of string. Instead we swap flag's current value with tentative value we just produced.

PiperOrigin-RevId: 285132636

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ed45d118fb818969eb13094cf7827c885dfc562c by Tom Manshreck <shreck@google.com>:

Change null-term* (and nul-term*) to NUL-term* in comments

PiperOrigin-RevId: 285036610

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729619017944db895ce8d6d29c1995aa2e5628a5 by Derek Mauro <dmauro@google.com>:

Use the Posix implementation of thread identity on MinGW.
Some versions of MinGW suffer from thread_local bugs.

PiperOrigin-RevId: 285022920

--
39a25493503c76885bc3254c28f66a251c5b5bb0 by Greg Falcon <gfalcon@google.com>:

Implementation detail change.

Add further ABSL_NAMESPACE_BEGIN and _END annotation macros to files in Abseil.

PiperOrigin-RevId: 285012012
GitOrigin-RevId: c99f979ad34f155fbeeea69b88bdc7458d89a21c
Change-Id: I4c85d3704e45d11a9ac50d562f39640a6adbedc1

absl/types/internal/conformance_aliases.h

@@ -26,6 +26,7 @@
 #include "absl/types/internal/conformance_profile.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace types_internal {
 
 // Creates both a Profile and a corresponding Archetype with root name "name".
@@ -438,6 +439,7 @@ using ExpandSupportedProfiles = Receiver<
 // (potentially) non-noexcept moves.
 
 }  // namespace types_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #undef ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS

absl/types/internal/conformance_archetype.h

@@ -43,6 +43,7 @@
 #include "absl/types/internal/conformance_profile.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace types_internal {
 
 // A minimum-conforming implementation of a type with properties specified in
@@ -961,6 +962,7 @@ struct EnabledHash {
 };
 
 }  // namespace types_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 namespace std {

absl/types/internal/conformance_profile.h

@@ -44,6 +44,7 @@
 // TODO(calabrese) Add support for extending profiles.
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 namespace types_internal {
 
 template <class T, class /*Enabler*/ = void>
@@ -369,6 +370,7 @@ template <class T>
 struct IsProfile : IsProfileImpl<T>::type {};
 
 }  // namespace types_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TYPES_INTERNAL_CONFORMANCE_PROFILE_H_

absl/types/internal/optional.h

@@ -54,6 +54,7 @@
 #endif
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // Forward declaration
 template <typename T>
@@ -387,6 +388,7 @@ struct optional_hash_base<T, decltype(std::hash<absl::remove_const_t<T> >()(
 };
 
 }  // namespace optional_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #undef ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS

absl/types/internal/span.h

@@ -26,6 +26,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 namespace span_internal {
 // A constexpr min function
@@ -121,6 +122,7 @@ template <typename From, typename To>
 using EnableIfConvertibleTo =
     typename std::enable_if<IsConvertible<From, To>::value>::type;
 }  // namespace span_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TYPES_INTERNAL_SPAN_H_

absl/types/internal/variant.h

@@ -40,6 +40,7 @@
 #if !defined(ABSL_USES_STD_VARIANT)
 
 namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 template <class... Types>
 class variant;
@@ -1638,6 +1639,7 @@ struct VariantHashBase<Variant,
 };
 
 }  // namespace variant_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // !defined(ABSL_USES_STD_VARIANT)