Parse tag names as well as expressions. The former is part of the
specification, the latter is for improved compatibility with the GNU assembler.
Fix attribute value handling to be comformant to the specification.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198662 91177308-0d34-0410-b5e6-96231b3b80d8
