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/lib.rs

@@ -16,7 +16,7 @@
 #![allow(internal_features)]
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]
-#![feature(rustc_private, decl_macro, never_type, trusted_len, hash_raw_entry, let_chains)]
+#![feature(rustc_private, decl_macro, never_type, trusted_len, let_chains)]
 #![allow(broken_intra_doc_links)]
 #![recursion_limit = "256"]
 #![warn(rust_2018_idioms)]