rust-lang#295: Use upstream version of the monomorphizer (rust-lang#495)

Instead of keeping our own copy of the monomorphizer, we would like to
start using rustc monomorphizer as is. This is the first step towards
that goal.

For now, we will use the upstream version of the monomorphizer with a
few small changes. Use a query to skip monomorphization for rmc instead
of hooks.

This change, adds a query to allow RMC to skip monomorphizing functions. It also does the following:
- Adds a default query to codegen base that don't skip anything.
- Adds a query implementation to RMC that checks our hooks.
- Skips monomorphizing the start function if --cfg=rmc

Issue rust-lang#485 tracks the rest of the changes needed.

Author: celinval

compiler/rustc_codegen_llvm/src/gotoc/mir_to_goto/compiler_interface.rs

@@ -5,7 +5,7 @@
 
 use crate::gotoc::cbmc::goto_program::symtab_transformer;
 use crate::gotoc::cbmc::goto_program::SymbolTable;
-use crate::gotoc::mir_to_goto::monomorphize;
+use crate::gotoc::mir_to_goto::overrides::skip_monomorphize;
 use crate::gotoc::mir_to_goto::GotocCtx;
 use bitflags::_core::any::Any;
 use rustc_codegen_ssa::traits::CodegenBackend;
@@ -42,7 +42,7 @@ impl CodegenBackend for GotocCodegenBackend {
     }
 
     fn provide(&self, providers: &mut Providers) {
-        monomorphize::partitioning::provide(providers);
+        providers.skip_monomorphize = skip_monomorphize;
     }
 
     fn provide_extern(&self, _providers: &mut ty::query::Providers) {}

compiler/rustc_codegen_llvm/src/gotoc/mir_to_goto/mod.rs

@@ -6,7 +6,6 @@
 mod codegen;
 mod compiler_interface;
 mod context;
-mod monomorphize;
 mod overrides;
 mod utils;