Use stable metric for const eval limit instead of current terminator-based logic
This patch adds a `MirPass` that inserts a new MIR instruction `ConstEvalCounter` to any loops and function calls in the CFG. This instruction is used during Const Eval to count against the `const_eval_limit`, and emit the `StepLimitReached` error, replacing the current logic which uses Terminators only.
The new method of counting loops and function calls should be more stable across compiler versions (i.e., not cause crates that compiled successfully before, to no longer compile when changes to the MIR generation/optimization are made).
Also see: #103877
Emit a single error for contiguous sequences of unknown tokens
Closes#106101
On encountering a sequence of identical source characters which are unknown tokens, note the amount of subsequent characters and advance past them silently. The old behavior was to emit an error and 'help' note for every single one.
`@rustbot` label +A-diagnostics +A-parser
