Export of internal Abseil changes

--
d857e6e1f9b09a3eb5abd890677a98b23346f07a by Abseil Team <absl-team@google.com>:

Simplify internal TryAcquireWithSpinning.

No point declaring the `result` variable: we can just return the results
directly.

PiperOrigin-RevId: 307045800

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421952252bc23be51f47f7d23f3422bad1ed382c by Derek Mauro <dmauro@google.com>:

Add custom sink support for `absl::Format()` through an ADL extension mechanism.

Users can now define
`void AbslFormatFlush(MySink* dest, absl::string_view part)`
to allow `absl::Format()` to append to a custom sink.

PiperOrigin-RevId: 306929052

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c73d5cdb62cd58ea421ed1aeeab78a0ffcfeeefb by Matt Calabrese <calabrese@google.com>:

Internal-only conformance-testing macro ABSL_INTERNAL_ASSERT_CONFORMANCE_OF for compile-time and runtime checks of a specified type, expected properties of that type, and a logically-ordered series of equivalence classes of that type.

PiperOrigin-RevId: 306885512

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a8c2495a07f37d68907855e3f0535bd5c27a3b52 by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 306766753
GitOrigin-RevId: d857e6e1f9b09a3eb5abd890677a98b23346f07a
Change-Id: Ic23c92ac74f9ffcbb2471ff8c6691f4b7b20354b

1 files changed, 34 insertions, 0 deletions

diff --git a/absl/types/internal/parentheses.h b/absl/types/internal/parentheses.h
new file mode 100644
index 00000000..5aebee8f
--- /dev/null
+++ b/absl/types/internal/parentheses.h
@@ -0,0 +1,34 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// parentheses.h
+// -----------------------------------------------------------------------------
+//
+// This file contains macros that expand to a left parenthesis and a right
+// parenthesis. These are in their own file and are generated from macros
+// because otherwise clang-format gets confused and clang-format off directives
+// do not help.
+//
+// The parentheses macros are used when wanting to require a rescan before
+// expansion of parenthesized text appearing after a function-style macro name.
+
+#ifndef ABSL_TYPES_INTERNAL_PARENTHESES_H_
+#define ABSL_TYPES_INTERNAL_PARENTHESES_H_
+
+#define ABSL_INTERNAL_LPAREN (
+
+#define ABSL_INTERNAL_RPAREN )
+
+#endif  // ABSL_TYPES_INTERNAL_PARENTHESES_H_