Changes imported from Abseil "staging" branch:

  - 5923451fb3b082e8bedb800cb676378b0f52d651 Remove internal-only deprecated APIs. by Daniel Katz <katzdm@google.com>
  - c715bf6e5533a9a5d827e806ccd6e8ee68ad2a53 Small fix for comment in span.h by Abseil Team <absl-team@google.com>
  - ef89cc8dac0631b4ad3499d1f0883670b43567df Rename an internal detail to de-conflict with a badly-nam... by Abseil Team <absl-team@google.com>
  - b53761a945ffdab39d5340904ca822571672f11a Remove base/internal/log_severity.cc, which is omitted fr... by Abseil Team <absl-team@google.com>
  - 56685b1852840d3838e24d83849d56644949e9b7 Reimplementing MallocHook such that the C API wraps the C... by Abseil Team <absl-team@google.com>

GitOrigin-RevId: 5923451fb3b082e8bedb800cb676378b0f52d651
Change-Id: I9b854d46b57990c9a10971391d762b280488bcee