Implement the rest of the SEH directive-parsing methods in the COFFAsmParser.

Implement the rest of the SEH directive-parsing methods in the COFFAsmParser.

Add a size alignment check to the .seh_stackalloc directive parser. Add a
more descriptive error message to the .seh_handler directive parser.

Add methods to the TargetAsmInfo struct in support of all this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131992 91177308-0d34-0410-b5e6-96231b3b80d8