build: switch to clang as the default compiler
build: switch to clang as the default compiler

Clang brings us working support for coroutines, which are
needed for Raft and for code simplification.

perf_simple_query as well as full system tests show no
significant performance regression.

Test: unit(dev, release, debug)

Closes #7531

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla