add a new driver-level -ferror-limit=412 option, which causes clang to stop

emitting diagnostics after it has produced that many errors.  Give this a
default value of 20 which produces plenty of errors for people to fix before
recompiling but not so many that their entire console scrolls away when the
compiler gets confused.  The experience looks like this:

$ clang foo.c
<tons of crap>
foo.c:102:3: error: unknown type name 'somethingbad'
  somethingbad x;
  ^
fatal error: too many errors emitted, stopping now
36 warnings and 20 errors generated.

llvm-svn: 100689