Fewer early errors.

`build_session` is passed an `EarlyErrorHandler` and then constructs a
`Handler`. But the `EarlyErrorHandler` is still used for some time after
that.

This commit changes `build_session` so it consumes the passed
`EarlyErrorHandler`, and also drops it as soon as the `Handler` is
built. As a result, `parse_cfg` and `parse_check_cfg` now take a
`Handler` instead of an `EarlyErrorHandler`.

compiler/rustc_session/src/errors.rs

@@ -444,3 +444,9 @@ pub(crate) struct FunctionReturnRequiresX86OrX8664;
 #[derive(Diagnostic)]
 #[diag(session_function_return_thunk_extern_requires_non_large_code_model)]
 pub(crate) struct FunctionReturnThunkExternRequiresNonLargeCodeModel;
+
+#[derive(Diagnostic)]
+#[diag(session_failed_to_create_profiler)]
+pub struct FailedToCreateProfiler {
+    pub err: String,
+}