fix Issue 12979 - Nothrow violation error is hidden by inline assembler

- warn->deprecate->error on unattributed asm in functions

- add explicit attributes on asm statments (unverified by compiler)

  asm pure nothrow @nogc @trusted {
    //...
  }

- added new AsmCompoundStatement because the parser
  already splits asm blocks into many AsmStatements