Track kind of closure in upvar categorization

Keep track of the kind of closure responsible for an upvar
Categorize upvars in `Fn` unboxed closures as freely aliasable

This causes borrowck to correctly reject mutation or mutable borrows
of upvars in `Fn` unboxed closures since the closure environment is
aliasable.

This also tracks the responsible closure in the aliasability
information returned and uses it to give a helpful diagnostic.

Closes issue #17780
Fix unit test that was illegally mutating an upvar
Add regression test for issue #17780
Rename RELEASES.txt to RELEASES.md. It's markdown.
Update RELEASES.md for 0.12.0
Update AUTHORS.txt for 0.12.0
Update per feedback
Update html_root_url for 0.12.0 release
More relnotes tweaks
Fix unclear macros documentation.
debuginfo: Add LLDB version handling to test infrastructure.
debuginfo: Don't mark struct fields as artificial.

LLDB doesn't allow for reading 'artifical' fields (fields that are generated by the compiler). So do not mark, slice fields, enum discriminants, and GcBox value fields as artificial.
saner parameter order for reallocation functions

Using reallocate(old_ptr, old_size, new_size, align) makes a lot more
sense than reallocate(old_ptr, new_size, align, old_size) and matches up
with the order used by existing platform APIs like mremap.

Closes #17837

[breaking-change]
add {Imm,M}utableIntSlice
remove crate_id attribute, add crate_name one

this is true as of https://github.com/rust-lang/rust/pull/15319
add mention of test attribute

Fixes #16413
auto merge of #17748 : mahkoh/rust/int_slice, r=aturon
auto merge of #17867 : jbcrail/rust/unclear-macros-doc, r=alexcrichton

I rearranged one sentence in the macros guide to make it less awkward.
Clean up the implementations of Bitv and BitvSet.

Functions that add bits now ensure that any unused bits are set to 0.
`into_bitv` sanitizes the nbits of the Bitv/BitvSet it returns by setting the nbits to the current capacity.
Fix a bug with `union_with` and `symmetric_difference` with due to not updating nbits properly
Add test cases to the _with functions
Remove `get_mut_ref`

This is a [breaking-change]. The things you will need to fix are:

1. BitvSet's `unwrap()` has been renamed to `into_bitv`
2. BitvSet's `get_mut_ref()` has been removed. Use `into_bitv()` and `from_bitv()` instead.