Add x86_64-asan-windows-msvc tier 3 target

[eholk]

This is a companion to #149644 and adds a target that can be used to enable AddressSanitizer by default on Windows.

• A tier 3 target must have a designated developer or developers (the "target
  maintainers") on record to be CCed when issues arise regarding the target.

I and the other maintainers of the Windows MSVC targets will be responsible for this target.

(cc @ChrisDenton @dpaoliello @Fulgen301 @lambdageek @sivadeilra @wesleywiser)

• Targets must use naming consistent with any existing targets; for instance, a
  target for the same CPU or OS as an existing Rust target should use the same
  name for that CPU or OS. Targets should normally use the same names and
  naming conventions as used elsewhere in the broader ecosystem beyond Rust
  (such as in other toolchains), unless they have a very good reason to
  diverge. Changing the name of a target can be highly disruptive, especially
  once the target reaches a higher tier, so getting the name right is important
  even for a tier 3 target.

This target follows the naming conventions agreed on for the similar x86_64-asan-linux-gnu target.

• Tier 3 targets may have unusual requirements to build or use, but must not
  create legal issues or impose onerous legal terms for the Rust project or for
  Rust developers or users.

This PR introduces no new legal terms or conditions.

• Neither this policy nor any decisions made regarding targets shall create any
  binding agreement or estoppel by any party. If any member of an approving
  Rust team serves as one of the maintainers of a target, or has any legal or
  employment requirement (explicit or implicit) that might affect their
  decisions regarding a target, they must recuse themselves from any approval
  decisions regarding the target's tier status, though they may otherwise
  participate in discussions.

Understood.

• Tier 3 targets should attempt to implement as much of the standard libraries
  as possible and appropriate (core for most targets, alloc for targets
  that can support dynamic memory allocation, std for targets with an
  operating system or equivalent layer of system-provided functionality), but
  may leave some code unimplemented (either unavailable or stubbed out as
  appropriate), whether because the target makes it impossible to implement or
  challenging to implement. The authors of pull requests are not obligated to
  avoid calling any portions of the standard library on the basis of a tier 3
  target not implementing those portions.

This is able to implement the same portions of the standard library that are supported by the other MSVC targets.

• The target must provide documentation for the Rust community explaining how
  to build for the target, using cross-compilation if possible. If the target
  supports running binaries, or running tests (even if they do not pass), the
  documentation must explain how to run such binaries or tests for the target,
  using emulation if possible or dedicated hardware if necessary.

This PR includes such documentation.

• Tier 3 targets must not impose burden on the authors of pull requests, or
  other developers in the community, to maintain the target. In particular,
  do not post comments (automated or manual) on a PR that derail or suggest a
  block on the PR based on a tier 3 target. Do not send automated messages or
  notifications (via any medium, including via @) to a PR author or others
  involved with a PR regarding a tier 3 target, unless they have opted into
  such messages.

Understood.

• Patches adding or updating tier 3 targets must not break any existing tier 2
  or tier 1 target, and must not knowingly break another tier 3 target without
  approval of either the compiler team or the maintainers of the other tier 3
  target.

Understood.

• Tier 3 targets must be able to produce assembly using at least one of
  rustc's supported backends from any host target. (Having support in a fork
  of the backend is not sufficient, it must be upstream.)

This target uses LLVM for code generation.

[rustbot]

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

These commits modify compiler targets.
(See the Target Tier Policy.)

[rustbot]

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[wesleywiser]

r? wesleywiser

I'll wait for a bit to see if there are any objections from the other target maintainers against adding this but I'm very much in favor!

[wesleywiser]

nit: let's make sure this ends up grouped with x86_64-asan-unknown-gnu once #149644 lands.

[wesleywiser]

I was initially going to suggest we just fold this file into the existing windows-msvc.md file but there are some important things to note like this, so I guess it makes sense to keep it as a separate file. If other maintainers feel differently, feel free to comment.