Make codegen coverage_context optional, and check

Addresses Issue #78286

Libraries compiled with coverage and linked with out enabling coverage
would fail when attempting to add the library's coverage statements to
the codegen coverage context (None).

Now, if coverage statements are encountered while compiling / linking
with `-Z instrument-coverage` disabled, codegen will *not* attempt to
add code regions to a coverage map, and it will not inject the LLVM
instrprof_increment intrinsic calls.

compiler/rustc_codegen_ssa/src/traits/coverageinfo.rs

@@ -9,14 +9,18 @@ pub trait CoverageInfoMethods: BackendTypes {
 pub trait CoverageInfoBuilderMethods<'tcx>: BackendTypes {
     fn create_pgo_func_name_var(&self, instance: Instance<'tcx>) -> Self::Value;
 
+    /// Returns true if the counter was added to the coverage map; false if `-Z instrument-coverage`
+    /// is not enabled (a coverage map is not being generated).
     fn add_counter_region(
         &mut self,
         instance: Instance<'tcx>,
         function_source_hash: u64,
         id: CounterValueReference,
         region: CodeRegion,
-    );
+    ) -> bool;
 
+    /// Returns true if the expression was added to the coverage map; false if
+    /// `-Z instrument-coverage` is not enabled (a coverage map is not being generated).
     fn add_counter_expression_region(
         &mut self,
         instance: Instance<'tcx>,
@@ -25,7 +29,9 @@ pub trait CoverageInfoBuilderMethods<'tcx>: BackendTypes {
         op: Op,
         rhs: ExpressionOperandId,
         region: CodeRegion,
-    );
+    ) -> bool;
 
-    fn add_unreachable_region(&mut self, instance: Instance<'tcx>, region: CodeRegion);
+    /// Returns true if the region was added to the coverage map; false if `-Z instrument-coverage`
+    /// is not enabled (a coverage map is not being generated).
+    fn add_unreachable_region(&mut self, instance: Instance<'tcx>, region: CodeRegion) -> bool;
 }