Add 'no_default_flags' option for turning off all default test compile and link flags.

When 'no_default_flags' is true only flags passed using '--param=compile_flags'
and '--param=link_flags' will be used when compiling the tests. This option
can be used to run the test suite against libstdc++ and helps with all
unusual test suite configurations.

NOTE: sanitizer flags are still added when '--param=use_sanitizer' is used even
if '--param=no_default_flags' is given.

llvm-svn: 226322