Rollup merge of #105286 - willcrichton:maximal-hir-to-mir-coverage, r=cjgillot

Add -Z maximal-hir-to-mir-coverage flag

This PR adds a new unstable flag `-Z maximal-hir-to-mir-coverage` that changes the behavior of `maybe_lint_level_root_bounded`, pursuant to [a discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Mapping.20MIR.20to.20HIR). When enabled, this function will not search upwards for a lint root, but rather immediately return the provided HIR node ID. This change increases the granularity of the mapping between MIR locations and HIR nodes inside the `SourceScopeLocalData` data structures. This increase in granularity is useful for rustc consumers like [Flowistry](https://github.com/willcrichton/flowistry) that rely on getting source-mapping information about the MIR CFG that is as precise as possible.

A test `maximal_mir_to_hir_coverage.rs` has been added to verify that this flag does not break anything.

r? `@cjgillot`

cc `@gavinleroy`

compiler/rustc_session/src/options.rs

@@ -1382,6 +1382,9 @@ options! {
         "list the symbols defined by a library crate (default: no)"),
     macro_backtrace: bool = (false, parse_bool, [UNTRACKED],
         "show macro backtraces (default: no)"),
+    maximal_hir_to_mir_coverage: bool = (false, parse_bool, [TRACKED],
+        "save as much information as possible about the correspondence between MIR and HIR \
+        as source scopes (default: no)"),
     merge_functions: Option<MergeFunctions> = (None, parse_merge_functions, [TRACKED],
         "control the operation of the MergeFunctions LLVM pass, taking \
         the same values as the target option of the same name"),