coverage: Store the number of counters/expressions in function coverage info

Coverage codegen can now allocate arrays based on the number of
counters/expressions originally used by the instrumentor.

The existing query that inspects coverage statements is still used for
determining the number of counters passed to `llvm.instrprof.increment`. If
some high-numbered counters were removed by MIR optimizations, the instrumented
binary can potentially use less memory and disk space at runtime.

compiler/rustc_mir_transform/src/coverage/counters.rs

@@ -126,6 +126,14 @@ impl CoverageCounters {
         next
     }
 
+    pub(super) fn num_counters(&self) -> usize {
+        self.next_counter_id.as_usize()
+    }
+
+    pub(super) fn num_expressions(&self) -> usize {
+        self.next_expression_id.as_usize()
+    }
+
     fn set_bcb_counter(
         &mut self,
         bcb: BasicCoverageBlock,