Export of internal Abseil changes

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adcea274552c5671bdafae7f85797c49c9639b67 by Abseil Team <absl-team@google.com>:

change some const declarations to constexpr

PiperOrigin-RevId: 307516528

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f828d23bdd437f38ae52ba232fd44c477643ddaf by Derek Mauro <dmauro@google.com>:

Use "-lrt" instead of the resolved find_library result when linking
librt. find_library defaults to shared objects. Imports #665.

PiperOrigin-RevId: 307465639
GitOrigin-RevId: adcea274552c5671bdafae7f85797c49c9639b67
Change-Id: I8f562ef8f57f5ed55192936c764fa6532e3b8adb

absl/strings/cord.h

@@ -645,12 +645,12 @@ class Cord {
   // InlineRep holds either a tree pointer, or an array of kMaxInline bytes.
   // InlineRep holds either a tree pointer, or an array of kMaxInline bytes.
   class InlineRep {
   class InlineRep {
    public:
    public:
-    static const unsigned char kMaxInline = 15;
+    static constexpr unsigned char kMaxInline = 15;
     static_assert(kMaxInline >= sizeof(absl::cord_internal::CordRep*), "");
     static_assert(kMaxInline >= sizeof(absl::cord_internal::CordRep*), "");
     // Tag byte & kMaxInline means we are storing a pointer.
     // Tag byte & kMaxInline means we are storing a pointer.
-    static const unsigned char kTreeFlag = 1 << 4;
+    static constexpr unsigned char kTreeFlag = 1 << 4;
     // Tag byte & kProfiledFlag means we are profiling the Cord.
     // Tag byte & kProfiledFlag means we are profiling the Cord.
-    static const unsigned char kProfiledFlag = 1 << 5;
+    static constexpr unsigned char kProfiledFlag = 1 << 5;
 
 
     constexpr InlineRep() : data_{} {}
     constexpr InlineRep() : data_{} {}
     InlineRep(const InlineRep& src);
     InlineRep(const InlineRep& src);

absl/synchronization/internal/waiter.h

@@ -101,7 +101,7 @@ class Waiter {
 
 
   // How many periods to remain idle before releasing resources
   // How many periods to remain idle before releasing resources
 #ifndef THREAD_SANITIZER
 #ifndef THREAD_SANITIZER
-  static const int kIdlePeriods = 60;
+  static constexpr int kIdlePeriods = 60;
 #else
 #else
   // Memory consumption under ThreadSanitizer is a serious concern,
   // Memory consumption under ThreadSanitizer is a serious concern,
   // so we release resources sooner. The value of 1 leads to 1 to 2 second
   // so we release resources sooner. The value of 1 leads to 1 to 2 second