AsmParser: Use do{}while(false) in macros, NFC

`do { ... } while (false)` is standard macro etiquette for forcing
instantiations into a single statement and requiring a `;` afterwards,
making statement-like macros easier to reason about (and harder to use
incorrectly).

I'm about to modify the macros in `LexIdentifier()`.  I noticed that the
`KEYWORD` macro *does* follow the rule, so I thought I'd clean up the
other macros to match (otherwise might not be worth changing, since the
benefits of this pattern are fairly irrelevant here).

llvm-svn: 230095