[clang] [Driver] Disable default configs via envvar during testing

[clang] [Driver] Disable default configs via envvar during testing

Add support for a CLANG_NO_DEFAULT_CONFIG envvar that works like
the --no-default-config option when set to a non-empty value.  Use it
to disable loading system configuration files during the test suite
runs.

Configuration files can change the driver behavior in extensive ways,
and it is neither really possible nor feasible to account for or undo
the effects of even the most common configuration uses.  Therefore,
the most reasonable option seems to be to ignore configuration files
while running the majority of tests (with the notable exception of tests
for configuration file support).

Due to the diversity of ways that %clang is used in the test suite,
including using it to copy or symlink the clang executable, as well to
call -cc1 and -cc1as modes, it is not feasible to pass the explicit
options to disable config loading either.  Using an environment variable
has the advantage of being easily applied across the test suite
and easily unset for default configuration file loading tests.

Differential Revision: https://reviews.llvm.org/D134905