Update README.md

Added 64-bit Windows support to the list of platforms. Support for 64-bit Windows was added in Rust 0.12.0. See  https://mail.mozilla.org/pipermail/rust-dev/2014-October/011267.html for the release notes.
Teach libsyntax about `while let`
Desugar `while let` into `loop { match { ... } }`
Move `while let` behind `while_let` feature gate
Handle `while let` desugaring
Add tests for `while let`
Add `while let` to the reference

Closes #17687
Use explicit case rather than catch-all
auto merge of #17910 : taylorhutchison/rust/patch-1, r=alexcrichton

Added 64-bit Windows support to the list of platforms. Support for 64-bit Windows was added in Rust 0.12.0. See  https://mail.mozilla.org/pipermail/rust-dev/2014-October/011267.html for the release notes.
auto merge of #17733 : jgallagher/rust/while-let, r=alexcrichton

This is *heavily* based on `if let` (#17634) by @jakub- and @kballard

This should close #17687
Add a memoize! macro and use it throughout rustc
Change memoize!() to be a function instead