Make coverage counter IDs count up from 0, not 1

Operand types are now tracked explicitly, so there is no need to reserve ID 0
for the special always-zero counter.

As part of the renumbering, this change fixes an off-by-one error in the way
counters were counted by the `coverageinfo` query. As a result, functions
should now have exactly the number of counters they actually need, instead of
always having an extra counter that is never used.

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -470,7 +470,7 @@ TrivialTypeTraversalAndLiftImpls! {
     ::rustc_hir::Unsafety,
     ::rustc_target::asm::InlineAsmRegOrRegClass,
     ::rustc_target::spec::abi::Abi,
-    crate::mir::coverage::CounterValueReference,
+    crate::mir::coverage::CounterId,
     crate::mir::coverage::ExpressionId,
     crate::mir::coverage::MappedExpressionIndex,
     crate::mir::Local,