debuginfo: slices are DW_TAG_array_type's

[shua]

Rust debuginfo for unsized structs with embedded slice types is lacking, and it makes it difficult for debuggers to correctly print types like Rc<str> (eg #114195). Specifically, these three structs would have essentially the same debug info (modulo some names):

struct Foo { a: u32, s: str } // unsized
struct Bar { a: u32, s: [u8] } // unsized
struct Baz { a: u32, s: u8 }

This changes how the compiler generates debuginfo in this case, and I used existing compilers (gcc, clang) handling of C flexible member arrays as a reference.

[rustbot]

r? @davidtwco

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[shua]

another relevant thread from gdb bugtracker about rust's weird unsized type debug info and some related fixes in gdb: https://sourceware.org/bugzilla/show_bug.cgi?id=30330 . The type of change here (changing slice types to be array types with no count, similar to C's VLA/FMA) was discussed but I guess never implemented in rustc?

[rust-bors]

☔ The latest upstream changes (presumably #153304) made this pull request unmergeable. Please resolve the merge conflicts.

[davidtwco]

Could you also add a codegen-llvm test for the debuginfo?

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