NLL is missing struct field suggestion

Part of #52663.

This commit adds suggestions to change the definitions of fields in
struct definitions from immutable references to mutable references.

r? @nikomatsakis
cc @pnkfelix

Run debuginfo tests against rust-enabled lldb, when possible

If the rust-enabled lldb was built, then use it when running the
debuginfo tests.  Updating the lldb submodule was necessary as this
needed a way to differentiate the rust-enabled lldb, so I added a line
to the --version output.

This adds compiletest commands to differentiate between the
rust-enabled and non-rust-enabled lldb, as is already done for gdb.  A
new "rust-lldb" header directive is also added, but not used in this
patch; I plan to use it in #54004.

This updates all the tests.

Fix range literals borrowing suggestions

Fixes #54505. The compiler issued incorrect range borrowing suggestions (missing `()` around borrows of range literals). This was not correct syntax (see the issue for an example).

With changes in this PR, this is fixed for all types of `Range` literals.

Thanks again to @varkor and @estebank for their invaluable help and guidance.

r? @varkor

liballoc: mark str.to_owned() and String::from(&str) as #[inline].

Fixes #53681

Prepare miri engine for enforcing validity invariant during execution

In particular, make recursive checking of references optional, and add a `const_mode` parameter that says whether `usize` is allowed to contain a pointer. Also refactor validation a bit to be type-driven at the "leafs" (primitive types), and separately validate scalar layout to catch `NonNull` violations (which it did not properly validate before).

Fixes https://github.com/rust-lang/rust/issues/53826
Also fixes https://github.com/rust-lang/rust/issues/54751

r? @oli-obk

also less verbose logging

Also, make enum variant handling a bit nicer

We'll try ruling out undef later

Fixes #54751

This does not actually regress anything.  It would regress NonNull, but we
didn't handle that correctly previously either.

user annotations in patterns

Fixes https://github.com/rust-lang/rust/issues/54573

r? @pnkfelix

[NLL]  Improve closure region bound errors

Previously, we would report free region errors that originate from closure with the span of the closure and a "closure body requires ..." message. This is now updated to use a reason and span from inside the closure.

structured suggestions for unused-lifetimes lint

Regretfully, resolve_lifetime.rs is suffering from a bit of rightward-drift, but

![zero_life](https://user-images.githubusercontent.com/1076988/46253407-010e7880-c430-11e8-8faf-b9afc1405415.png)

r? @nikomatsakis

If the rust-enabled lldb was built, then use it when running the
debuginfo tests.  Updating the lldb submodule was necessary as this
needed a way to differentiate the rust-enabled lldb, so I added a line
to the --version output.

This adds compiletest commands to differentiate between the
rust-enabled and non-rust-enabled lldb, as is already done for gdb.  A
new "rust-lldb" header directive is also added, but not used in this
patch; I plan to use it in #54004.

This updates all the tests.

Remove unnecessary comma in `libstd/thread/mod.rs` doc comment

Fix typo in `libstd/thread/mod.rs` doc comment: remove unnecessary comma.

This commit changes the label to a maybe incorrect suggestion for better
integration with RLS.