Fuchsia doesn't support dirfd although we have a symbol stubbed for it.

…infallible, r=pnkfelix

Remove optionality from MoveData::base_local

This is an artifact from when Places could be based on statics and not just locals. Now, all move paths either are locals or have parents, so this doesn't need to return Option anymore.

… r=pnkfelix

Add a note to the ArbitraryExpressionInPattern error

The current "arbitrary expressions aren't allowed in patterns" error is confusing, as it fires for code where it *looks* like a pattern but the compiler still treats it as an expression. That this is due to the `:expr` fragment specifier forcing the expression-ness property on the code.

In the test suite, the "arbitrary expressions aren't allowed in patterns" error can only be found in combination with macro_rules macros that force expression-ness of their content, namely via `:expr` metavariables. I also can't come up with cases where there would be an expression instead of a pattern, so I think it's always coming from an `:expr`.

In order to make the error less confusing, this adds a note explaining the weird `:expr` fragment behaviour.

Fixes rust-lang#99380

Fix Fuchsia build broken by rust-lang#124210

Fuchsia doesn't support dirfd although we have a symbol stubbed for it.