Fix PR20069: bad loop pragma arguments crash FE

This patch fixes a crash when handling malformed arguments to loop pragmas such
as: "#pragma clang loop vectorize(()".  Essentially any argument which is not an
identifier or constant resulted in a crash.  This patch also changes a couple of
the error messages which weren't quite correct.  New behavior with this patch vs
old behavior:

#pragma clang loop vectorize(1)
OLD: error: missing keyword; expected 'enable' or 'disable'
NEW: error: invalid argument; expected 'enable' or 'disable'

#pragma clang loop vectorize()
OLD: error: expected ')'
NEW: error: missing argument to loop pragma 'vectorize'

#pragma clang loop vectorize_width(bad)
OLD: error: missing value; expected a positive integer value
NEW: error: invalid argument; expected a positive integer value

#pragma clang loop vectorize(bad)
OLD: invalid keyword 'bad'; expected 'enable' or 'disable'
NEW: error: invalid argument; expected 'enable' or 'disable'

http://reviews.llvm.org/D4197

Patch by Mark Heffernan