Add a --time-trace flag to configure.py and a Scylla_TIME_TRACE CMake
option that enable Clang's -ftime-trace on all C++ compilations. When
enabled, each .o file produces a companion .json trace that can be
analyzed with ClangBuildAnalyzer or loaded in chrome://tracing to
identify slow headers and costly template instantiations.
This is the first step toward data-driven build speed improvements.
Refs #1
Usage:
configure.py: ./configure.py --time-trace --mode dev
CMake: cmake -DScylla_TIME_TRACE=ON -DCMAKE_BUILD_TYPE=Dev ..
Closesscylladb/scylladb#29462