When determining whether an expression refers to a bit-field, look

When determining whether an expression refers to a bit-field, look
into the left-hand side of an assignment expression. This completes
most of PR3500; the only remaining part is to deal with the
GCC-specific implementation-defined behavior for "unsigned long" (and
other) bit-fields.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@70623 91177308-0d34-0410-b5e6-96231b3b80d8