Stop using hash_raw_entry in CodegenCx::const_str

That unstable feature completed fcp-close, so the compiler needs to be
migrated away to allow its removal. In this case, `cg_llvm` and `cg_gcc`
were using raw entries to optimize their `const_str_cache` lookup and
insertion. We can change that to separate `get` and (on miss) `insert`
calls, so we still have the fast path avoiding string allocation when
the cache hits.

compiler/rustc_codegen_gcc/src/common.rs

@@ -146,13 +146,12 @@ impl<'gcc, 'tcx> ConstCodegenMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
     }
 
     fn const_str(&self, s: &str) -> (RValue<'gcc>, RValue<'gcc>) {
-        let str_global = *self
-            .const_str_cache
-            .borrow_mut()
-            .raw_entry_mut()
-            .from_key(s)
-            .or_insert_with(|| (s.to_owned(), self.global_string(s)))
-            .1;
+        let mut const_str_cache = self.const_str_cache.borrow_mut();
+        let str_global = const_str_cache.get(s).copied().unwrap_or_else(|| {
+            let g = self.global_string(s);
+            const_str_cache.insert(s.to_owned(), g);
+            g
+        });
         let len = s.len();
         let cs = self.const_ptrcast(
             str_global.get_address(None),