[BUILD] Provide LIKELY / UNLIKELY macros

[marcalff]

Contributes to #2572

Changes

Please provide a brief description of the changes here.

• Remove the OPENTELEMETRY_LIKELY_IF macro
  • This macro does not make sense because it includes the if itself, making it unusable for other constructs (for loops, while loops)
• Provide a OPENTELEMETRY_LIKELY_CONDITION macro
  • To be used instead of OPENTELEMETRY_LIKELY_IF.
  • Only one argument is supported instead of a __VA_ARGS__, to discourage if OPENTELEMETRY_LIKELY_CONDITION (a = foo(), b = bar(), a > b)
  • This macro supports both compiler builtins for gcc and clang, and C++20.
• Provide a OPENTELEMETRY_UNLIKELY_CONDITION macro
• Provide a OPENTELEMETRY_LIKELY macro
• Provide a OPENTELEMETRY_UNLIKELY macro

With this change, likely / unlikely hints can be placed anywhere (for loops, while loops, switch case, etc). These hints are not used only for if conditions.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[owent]

LGTM and thanks.