Introduce a TargetFrontendConfig type. (#570)

* Introduce a `TargetFrontendConfig` type.

`TargetFrontendConfig` is information specific to the target which is
provided to frontends to allow them to produce Cranelift IR for the
target. Currently this includes the pointer size and the default calling
convention.

The default calling convention is now inferred from the target, rather
than being a setting. cranelift-native is now just a provider of target
information, rather than also being a provider of settings, which gives
it a clearer role.

And instead of having cranelift-frontend routines require the whole
`TargetIsa`, just require the `TargetFrontendConfig`, and add a way to
get the `TargetFrontendConfig` from a `Module`.

Fixes #529.
Fixes #555.

lib/wasm/src/environ/dummy.rs

@@ -5,13 +5,12 @@ use cranelift_codegen::cursor::FuncCursor;
 use cranelift_codegen::ir::immediates::{Imm64, Offset32};
 use cranelift_codegen::ir::types::*;
 use cranelift_codegen::ir::{self, InstBuilder};
-use cranelift_codegen::settings;
+use cranelift_codegen::isa::TargetFrontendConfig;
 use cranelift_entity::{EntityRef, PrimaryMap};
 use environ::{FuncEnvironment, GlobalVariable, ModuleEnvironment, ReturnMode, WasmResult};
 use func_translator::FuncTranslator;
 use std::string::String;
 use std::vec::Vec;
-use target_lexicon::Triple;
 use translation_utils::{
     DefinedFuncIndex, FuncIndex, Global, GlobalIndex, Memory, MemoryIndex, SignatureIndex, Table,
     TableIndex,
@@ -44,11 +43,8 @@ impl<T> Exportable<T> {
 /// `DummyEnvironment` to allow it to be borrowed separately from the
 /// `FuncTranslator` field.
 pub struct DummyModuleInfo {
-    /// Target description.
-    pub triple: Triple,
-
-    /// Compilation setting flags.
-    pub flags: settings::Flags,
+    /// Target description relevant to frontends producing Cranelift IR.
+    config: TargetFrontendConfig,
 
     /// Signatures as provided by `declare_signature`.
     pub signatures: PrimaryMap<SignatureIndex, ir::Signature>,
@@ -76,11 +72,10 @@ pub struct DummyModuleInfo {
 }
 
 impl DummyModuleInfo {
-    /// Allocates the data structures with the given flags.
-    pub fn with_triple_flags(triple: Triple, flags: settings::Flags) -> Self {
+    /// Creates a new `DummyModuleInfo` instance.
+    pub fn new(config: TargetFrontendConfig) -> Self {
         Self {
-            triple,
-            flags,
+            config,
             signatures: PrimaryMap::new(),
             imported_funcs: Vec::new(),
             functions: PrimaryMap::new(),
@@ -111,23 +106,10 @@ pub struct DummyEnvironment {
 }
 
 impl DummyEnvironment {
-    /// Allocates the data structures with default flags.
-    pub fn with_triple(triple: Triple) -> Self {
-        Self::with_triple_flags(
-            triple,
-            settings::Flags::new(settings::builder()),
-            ReturnMode::NormalReturns,
-        )
-    }
-
-    /// Allocates the data structures with the given triple.
-    pub fn with_triple_flags(
-        triple: Triple,
-        flags: settings::Flags,
-        return_mode: ReturnMode,
-    ) -> Self {
+    /// Creates a new `DummyEnvironment` instance.
+    pub fn new(config: TargetFrontendConfig, return_mode: ReturnMode) -> Self {
         Self {
-            info: DummyModuleInfo::with_triple_flags(triple, flags),
+            info: DummyModuleInfo::new(config),
             trans: FuncTranslator::new(),
             func_bytecode_sizes: Vec::new(),
             return_mode,
@@ -169,12 +151,8 @@ impl<'dummy_environment> DummyFuncEnvironment<'dummy_environment> {
 }
 
 impl<'dummy_environment> FuncEnvironment for DummyFuncEnvironment<'dummy_environment> {
-    fn triple(&self) -> &Triple {
-        &self.mod_info.triple
-    }
-
-    fn flags(&self) -> &settings::Flags {
-        &self.mod_info.flags
+    fn target_config(&self) -> TargetFrontendConfig {
+        self.mod_info.config
     }
 
     fn make_global(&mut self, func: &mut ir::Function, index: GlobalIndex) -> GlobalVariable {
@@ -348,8 +326,8 @@ impl<'dummy_environment> FuncEnvironment for DummyFuncEnvironment<'dummy_environ
 }
 
 impl<'data> ModuleEnvironment<'data> for DummyEnvironment {
-    fn flags(&self) -> &settings::Flags {
-        &self.info.flags
+    fn target_config(&self) -> &TargetFrontendConfig {
+        &self.info.config
     }
 
     fn get_func_name(&self, func_index: FuncIndex) -> ir::ExternalName {