[orc-rt] Add error.h: structured error support. (#154869)

Adds support for the Error class, Expected class template, and related
APIs that will be used for error propagation and handling in the new ORC
runtime.

The implementations of these types are cut-down versions of similar APIs
in llvm/Support/Error.h. Most advice on llvm::Error and llvm::Expected
(e.g. from the LLVM Programmer's manual) applies equally to
orc_rt::Error and orc_rt::Expected.

Ported from the old ORC runtime at compiler-rt/lib/orc.

Author: lhames

orc-rt/include/CMakeLists.txt

@@ -1,6 +1,8 @@
 set(ORC_RT_HEADERS
     orc-rt-c/orc-rt.h
     orc-rt/bitmask-enum.h
+    orc-rt/error.h
+    orc-rt/compiler.h
     orc-rt/math.h
     orc-rt/rtti.h
     orc-rt/span.h

orc-rt/include/orc-rt/compiler.h

@@ -0,0 +1,74 @@
+//===--------- compiler.h - Compiler abstraction support --------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file is a part of the ORC runtime support library.
+//
+// Most functionality in this file was swiped from llvm/Support/Compiler.h.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef ORC_RT_COMPILER_H
+#define ORC_RT_COMPILER_H
+
+#if defined(_WIN32)
+#define ORC_RT_INTERFACE extern "C"
+#define ORC_RT_HIDDEN
+#define ORC_RT_IMPORT extern "C" __declspec(dllimport)
+#else
+#define ORC_RT_INTERFACE extern "C" __attribute__((visibility("default")))
+#define ORC_RT_HIDDEN __attribute__((visibility("hidden")))
+#define ORC_RT_IMPORT extern "C"
+#endif
+
+#ifndef __has_builtin
+#define __has_builtin(x) 0
+#endif
+
+// Only use __has_cpp_attribute in C++ mode. GCC defines __has_cpp_attribute in
+// C mode, but the :: in __has_cpp_attribute(scoped::attribute) is invalid.
+#ifndef ORC_RT_HAS_CPP_ATTRIBUTE
+#if defined(__cplusplus) && defined(__has_cpp_attribute)
+#define ORC_RT_HAS_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
+#else
+#define ORC_RT_HAS_CPP_ATTRIBUTE(x) 0
+#endif
+#endif
+
+// Use the 'nodiscard' attribute in C++17 or newer mode.
+#if defined(__cplusplus) && __cplusplus > 201402L &&                           \
+    ORC_RT_HAS_CPP_ATTRIBUTE(nodiscard)
+#define ORC_RT_NODISCARD [[nodiscard]]
+#elif ORC_RT_HAS_CPP_ATTRIBUTE(clang::warn_unused_result)
+#define ORC_RT_NODISCARD [[clang::warn_unused_result]]
+// Clang in C++14 mode claims that it has the 'nodiscard' attribute, but also
+// warns in the pedantic mode that 'nodiscard' is a C++17 extension (PR33518).
+// Use the 'nodiscard' attribute in C++14 mode only with GCC.
+// TODO: remove this workaround when PR33518 is resolved.
+#elif defined(__GNUC__) && ORC_RT_HAS_CPP_ATTRIBUTE(nodiscard)
+#define ORC_RT_NODISCARD [[nodiscard]]
+#else
+#define ORC_RT_NODISCARD
+#endif
+
+#if __has_builtin(__builtin_expect)
+#define ORC_RT_LIKELY(EXPR) __builtin_expect((bool)(EXPR), true)
+#define ORC_RT_UNLIKELY(EXPR) __builtin_expect((bool)(EXPR), false)
+#else
+#define ORC_RT_LIKELY(EXPR) (EXPR)
+#define ORC_RT_UNLIKELY(EXPR) (EXPR)
+#endif
+
+#if defined(__APPLE__)
+#define ORC_RT_WEAK_IMPORT __attribute__((weak_import))
+#elif defined(_WIN32)
+#define ORC_RT_WEAK_IMPORT
+#else
+#define ORC_RT_WEAK_IMPORT __attribute__((weak))
+#endif
+
+#endif // ORC_RT_COMPILER_H