Rollup merge of #124555 - Zalathar:init-coverage, r=nnethercote

coverage: Clean up creation of MC/DC condition bitmaps

This PR improves the code for creating and initializing [MC/DC](https://en.wikipedia.org/wiki/Modified_condition/decision_coverage) condition bitmap variables, as introduced by #123409 and modified by #124255.

- The condition bitmap variables are now created eagerly at the start of per-function codegen, via a new `init_coverage` method in `CoverageInfoBuilderMethods`. This avoids having to retroactively create the bitmaps while doing codegen for an individual coverage statement.
- As a result, we can now create and initialize those bitmaps using existing safe APIs, instead of having to perform our own unsafe call to `llvm::LLVMBuildAlloca`.
- This PR also tweaks the way we count the number of condition bitmaps needed, by tracking the total number of bitmaps needed (max depth + 1), instead of only tracking the maximum depth. This reduces the potential for subtle off-by-one confusion.

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -259,6 +259,10 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // Apply debuginfo to the newly allocated locals.
     fx.debug_introduce_locals(&mut start_bx);
 
+    // If the backend supports coverage, and coverage is enabled for this function,
+    // do any necessary start-of-function codegen (e.g. locals for MC/DC bitmaps).
+    start_bx.init_coverage(instance);
+
     // The builders will be created separately for each basic block at `codegen_block`.
     // So drop the builder of `start_llbb` to avoid having two at the same time.
     drop(start_bx);

compiler/rustc_codegen_ssa/src/traits/coverageinfo.rs

@@ -3,6 +3,11 @@ use rustc_middle::mir::coverage::CoverageKind;
 use rustc_middle::ty::Instance;
 
 pub trait CoverageInfoBuilderMethods<'tcx>: BackendTypes {
+    /// Performs any start-of-function codegen needed for coverage instrumentation.
+    ///
+    /// Can be a no-op in backends that don't support coverage instrumentation.
+    fn init_coverage(&mut self, _instance: Instance<'tcx>) {}
+
     /// Handle the MIR coverage info in a backend-specific way.
     ///
     /// This can potentially be a no-op in backends that don't support