coverage: Attribute macro-generated function bodies to their invocation site

[alex]

Functions whose bodies are entirely proc-macro-generated (e.g. the impls from a #[derive(...)] macro) have every statement span pointing at the macro invocation site, because proc-macro output tokens get call-site spans.

Before Rust 1.84, the empty invocation-site spans that survived span refinement were widened by one column, so the invocation line reported the generated code's execution count via the entry counter. PR #132675 (1.84) restricted that widening to spans adjacent to '{' or '}', which as a side effect left such functions either completely uninstrumented, or with a single leftover sliver region wired to an arbitrary non-entry counter that reads 0 even when the function runs.

Restore the pre-1.84 semantics deliberately: when a function body is entirely macro-expanded and its spans point back into the invocation site (distinguishing proc-macro output from macro_rules! definition-site tokens, which normal refinement still handles), emit a single code region over the visible invocation site - the derive-macro path, the attribute, or name! - wired to the function's entry counter.

Never-called generated functions get the same region as an unused-function mapping (count 0); executed siblings from the same invocation share the span, so llvm-cov's aggregation covers the line. #[automatically_derived] impls remain uninstrumented, as before.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in coverage tests.

cc @Zalathar

Some changes occurred in coverage instrumentation.

cc @Zalathar

[rustbot]

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[alex]

@Zalathar FYI, this undoes one of the consequences of #132675. I don't think it regresses though :-)