Change .thumb_set to have the same error checks as .set.

Change .thumb_set to have the same error checks as .set.

According to the documentation, .thumb_set is 'the equivalent of a .set directive'.

We didn't have equivalent behaviour in terms of all the errors we could throw, for
example, when a symbol is redefined.

This change refactors parseAssignment so that it can be used by .set and .thumb_set
and implements tests for .thumb_set for all the errors thrown by that method.

Reviewed by Rafael Espíndola.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240318 91177308-0d34-0410-b5e6-96231b3b80d8