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_llvm/src/context.rs

@@ -324,8 +324,8 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
     }
 
     #[inline]
-    pub fn coverage_context(&'a self) -> &'a coverageinfo::CrateCoverageContext<'tcx> {
-        self.coverage_cx.as_ref().unwrap()
+    pub fn coverage_context(&'a self) -> Option<&'a coverageinfo::CrateCoverageContext<'tcx>> {
+        self.coverage_cx.as_ref()
     }
 }