Add a lint for `asm` blocks that should be marked volatile

[jeremydavis519]

While we shouldn't make it impossible to write non-volatile ASM (maybe the programmer wants to allow compiler optimizations), we should give a warning whenever it looks like the compiler might elide an asm block.

According to https://gcc.gnu.org/onlinedocs/gcc-4.7.2/gcc/Extended-Asm.html, a non-volatile asm block might be elided by GCC if

• "all the output operands you specify are for th[e] purpose [of clobbering inputs] (and hence unused)"
• "the memory affected [by a clobber] is not listed in the inputs or outputs of the asm, as the 'memory' clobber does not count as a side-effect of the asm"

One exception: "An asm instruction without any output operands will be treated identically to a volatile asm instruction." So no warning is needed in that case.

It's not clear to me right now how LLVM's rules for optimizing inline ASM compare to GCC's rules. I'll have to look into it before adding the lint.

[jeremydavis519]

Also, since we can't actually tell when the logic of a particular program won't require a volatile asm block, we can't reasonably make the opposite lint (i.e. a warning when "volatile" is used unnecessarily). There would need to be a way to opt out of the warning, and every way to do that seems too verbose.