ARM: improve diagnostics for .word directive

If a complex expression was passed to the .word directive and the first part of
the directive failed to parse, a secondary diagnostic would be produced that
would clutter the error diagnostics.  Improve the diagnostics by consuming the
remainder of the statement.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200160 91177308-0d34-0410-b5e6-96231b3b80d8