[flang] Make runtimes headers independent of the main llvm headers

[mstorsjo]

Summary

llvm/Support/Compiler.h requires llvm-config.h, i.e. to include it,
you'd need to actually set up building llvm for that target.

If using flang to cross compile code for a different target, we need
the runtimes built for that target, but we might not want to build
llvm itself for that target.

This is one out of two essential steps for building the flang runtimes
for a foreign target, i.e. setting up cross compiling with flang.

[clementval]

flang/include/flang/Common/idioms.h

37

[mstorsjo]

Applied the suggested style fix

[rovka]

flang/include/flang/Common/idioms.h

44

Nitpick: Can we just use C++17's maybe_unused attribute instead? (This header is already requiring C++17)

[mstorsjo]

Use C++17 [[maybe_unused]].

flang/include/flang/Common/idioms.h

44

Thanks, that’s much nicer!

[mstorsjo]

Ping @rovka - does this seem ok now?

[mstorsjo]

Ping @rovka

[mstorsjo]

@rovka Ping

[mstorsjo]

Adding a few more reviewers here - is someone of you willing to have a look at this?

[mstorsjo]

Third weekly ping - @rovka @awarzynski @clementval.

[clementval]

LGTM. Please wait on @rovka as well.

[awarzynski]

Sorry about the delay ("holiday season"), makes sense to me. Thanks for contributing!

You've already addressed Diana's comment and have "+1" from 2 other reviewers. I would just land it if there are no new comments today.

flang/include/flang/Common/idioms.h

 #if __cplusplus < 201703L
 #error this is a C++17 program
 #endif
 #if !__clang__ && defined __GNUC__ && __GNUC__ < 7
 #error g++ >= 7.2 is required
 #endif
 
 #include "visit.h"
-#include "llvm/Support/Compiler.h"
 #include <functional>
 #include <list>
 #include <memory>
 #include <optional>
 #include <string>
 #include <tuple>
 #include <type_traits>
 #include <variant>
 
 #if __GNUC__ == 7
 // Avoid a deduction bug in GNU 7.x headers by forcing the answer.
 namespace std {
 template <typename A>
 struct is_trivially_copy_constructible<list<A>> : false_type {};
 template <typename A>
 struct is_trivially_copy_constructible<optional<list<A>>> : false_type {};
 } // namespace std
 #endif
 
 // enable "this is a std::string"s with the 's' suffix
 using namespace std::literals::string_literals;
 
 namespace Fortran::common {
 
 // Helper templates for combining a list of lambdas into an anonymous
 // struct for use with common::visit() on a std::variant<> sum type.

 
 template <typename A> struct ListItemCount {
   constexpr ListItemCount(std::initializer_list<A> list) : value{list.size()} {}
   const std::size_t value;
 };
 
 #define ENUM_CLASS(NAME, ...) \
   enum class NAME { __VA_ARGS__ }; \
-  LLVM_ATTRIBUTE_UNUSED static constexpr std::size_t NAME##_enumSize{[] { \
+  [[maybe_unused]] static constexpr std::size_t NAME##_enumSize{[] { \
     enum { __VA_ARGS__ }; \
     return Fortran::common::ListItemCount{__VA_ARGS__}.value; \
   }()}; \
-  LLVM_ATTRIBUTE_UNUSED static inline std::string EnumToString(NAME e) { \
+  [[maybe_unused]] static inline std::string EnumToString(NAME e) { \
     return Fortran::common::EnumIndexToString( \
         static_cast<int>(e), #__VA_ARGS__); \
   }
 
 // Check that a pointer is non-null and dereference it
 #define DEREF(p) Fortran::common::Deref(p, __FILE__, __LINE__)
 
 template <typename T> constexpr T &Deref(T *p, const char *file, int line) {