asm!: lint on bare memory accesses without [] or offset

[joshtriplett]

In Intel syntax mode without prefixes, writing mov rax, some_symbol is equivalent to mov rax, [some_symbol]: in both cases it refers to the memory pointed to by the symbol. However, I've seen multiple people think the former syntax obtains the address of the symbol instead. (Referring to the address of the symbol requires offset, which doesn't currently work in LLVM.) This confusion can result in bugs that are quite difficult to debug.

I think it'd be appropriate, working in conjunction with a future version of LLVM's assembly parser, to catch "bare" uses of symbols, and suggest either adding [] around it to access the memory pointed to by the symbol, or adding offset to use the address of the symbol.