Emit -verify diagnostics even when we have a fatal error.

Previously we'd halt at the fatal error as expected, but not actually emit
any -verify-related diagnostics. This lets us catch cases that emit a
/different/ fatal error from the one we expected.

This is implemented by adding a "force emit" mode to DiagnosticBuilder, which
will cause diagnostics to immediately be emitted regardless of current
suppression. Needless to say this should probably be used /very/ sparingly.

Patch by Andy Gibbs! Tests for all of Andy's -verify patches coming soon.

llvm-svn: 160053