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!`.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.