Rollup merge of #132124 - Zalathar:consolidate-covstar, r=jieyouxu

coverage: Consolidate creation of covmap/covfun records

This code for creating covmap/covfun records during codegen was split across multiple functions and files for dubious historical reasons. Having it all in one place makes it easier to follow.

This PR also includes two semi-related cleanups:
- Getting the codegen context's `coverage_cx` state is made infallible, since it should always exist when running the code paths that need it.
- The value of `covfun_section_name` is saved in the codegen context, since it never changes at runtime, and the code that needs it has access to the context anyway.

---

Background: Coverage instrumentation generates two kinds of metadata that are embedded in the final binary. There is per-CGU information that goes in the `__llvm_covmap` linker section, and per-function information that goes in the `__llvm_covfun` section (except on Windows, where slightly different section names are used).

compiler/rustc_codegen_llvm/src/context.rs

@@ -80,6 +80,7 @@ pub(crate) struct CodegenCx<'ll, 'tcx> {
 
     pub isize_ty: &'ll Type,
 
+    /// Extra codegen state needed when coverage instrumentation is enabled.
     pub coverage_cx: Option<coverageinfo::CrateCoverageContext<'ll, 'tcx>>,
     pub dbg_cx: Option<debuginfo::CodegenUnitDebugContext<'ll, 'tcx>>,
 
@@ -592,11 +593,10 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
         &self.statics_to_rauw
     }
 
+    /// Extra state that is only available when coverage instrumentation is enabled.
     #[inline]
-    pub(crate) fn coverage_context(
-        &self,
-    ) -> Option<&coverageinfo::CrateCoverageContext<'ll, 'tcx>> {
-        self.coverage_cx.as_ref()
+    pub(crate) fn coverage_cx(&self) -> &coverageinfo::CrateCoverageContext<'ll, 'tcx> {
+        self.coverage_cx.as_ref().expect("only called when coverage instrumentation is enabled")
     }
 
     pub(crate) fn create_used_variable_impl(&self, name: &'static CStr, values: &[&'ll Value]) {