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/mod.rs

@@ -214,6 +214,8 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
 
         self.mir_body.function_coverage_info = Some(Box::new(FunctionCoverageInfo {
             function_source_hash: self.function_source_hash,
+            num_counters: self.coverage_counters.num_counters(),
+            num_expressions: self.coverage_counters.num_expressions(),
         }));
     }