Automatically enable version-dependent GHC flags

Some GHC flags are very useful, but only appear in more recent GHC
versions. To support the use of such flags while still supporting
older compilers, let's add conditional checks and enabling based on
the results.

Currently only `-fwarn-incomplete-uni-patterns` is enabled, which
detects refutable patterns in lambda expressions and pattern bindings
(e.g. "let (Just x) = y"); such constructs are bad as they can lead to
runtime exceptions.

Additionally, fix an existing such bad construct in a test case; we
workaround it by using error, since that should never fail.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>