Rename asm! to llvm_asm!
As per https://github.com/rust-lang/rfcs/pull/2843, this PR renames `asm!` to `llvm_asm!`. It also renames the compiler's internal `InlineAsm` data structures to `LlvmInlineAsm` in preparation for the new `asm!` functionality specified in https://github.com/rust-lang/rfcs/pull/2850.
This PR doesn't actually deprecate `asm!` yet, it just makes it redirect to `llvm_asm!`. This is necessary because we first need to update the submodules (in particular stdarch) to use `llvm_asm!`.