Initial support for aliases. (#1262)

This supports aliases for types (including interfaces), functions, parameterized types, instance member names, and interface member names.

Co-authored-by: Jon Meow <jperkins@google.com>

explorer/testdata/alias/function_alias.carbon

@@ -0,0 +1,24 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 7
+
+package ExplorerTest api;
+
+alias TypeAlias = i32;
+
+fn Function(a: i32, b: TypeAlias) -> i32 { return a + b; }
+fn GenericFunction[T:! Type](x: T) -> T { return x; }
+
+alias FunctionAlias = Function;
+alias GenericFunctionAlias = GenericFunction;
+
+fn Main() -> i32 {
+  return FunctionAlias(1, 2) + GenericFunctionAlias(4);
+}