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Allow the `asm!` macro to accept a series of template arguments, and
interpret them as if they were concatenated with a '\n' between them.
This allows writing an `asm!` where each line of assembly appears in a
separate template string argument.
This syntax makes it possible for rustfmt to reliably format and indent
each line of assembly, without risking changes to the inside of a
template string. It also avoids the complexity of having the user
carefully format and indent a multi-line string (including where to put
the surrounding quotes), and avoids the extra indentation and lines of a
call to `concat!`.
For example, rewriting the second example from the [blog post on the new
inline assembly
syntax](https://blog.rust-lang.org/inside-rust/2020/06/08/new-inline-asm.html)
using multiple template strings:
```rust
fn main() {
let mut bits = [0u8; 64];
for value in 0..=1024u64 {
let popcnt;
unsafe {
asm!(
" popcnt {popcnt}, {v}",
"2:",
" blsi rax, {v}",
" jz 1f",
" xor {v}, rax",
" tzcnt rax, rax",
" stosb",
" jmp 2b",
"1:",
v = inout(reg) value => _,
popcnt = out(reg) popcnt,
out("rax") _, // scratch
inout("rdi") bits.as_mut_ptr() => _,
);
}
println!("bits of {}: {:?}", value, &bits[0..popcnt]);
}
}
```
Note that all the template strings must appear before all other
arguments; you cannot, for instance, provide a series of template
strings intermixed with the corresponding operands.
In order to get srcloc mappings right for macros that generate
multi-line string literals, create one line_span for each
line in the string literal, each pointing to the macro.
Make `rustc_parse_format::Parser::curarg` `pub`, so that we can
propagate it from one template string argument to the next.
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