Don't use '-fbuild-session-timestamp' if
'-disable-modules-validate-system-headers' is specified.
Since system header validation happens under *OR* condition of
'ValidateSystemInputs' and 'ModulesValidateOncePerBuildSession', using
build sessions effectively overrides it.
rdar://problem/61075677
Pass '-fbuild-session-timestamp' and '-fmodules-validate-once-per-build-sessio'
to ClangImporter so that module validation happens only once for the
SourceKit lifetime.
rdar://problem/59567281