[flang] Fix merge issue with memcpy and address build warnings

  - Fixes unresolved memcpy error seen in:
      smoke-fort-dev/{flang-464660-2,rocm-issue-201}

Author: dpalermo

flang-rt/include/flang-rt/runtime/descriptor.h

@@ -29,8 +29,10 @@
 #include <cstdio>
 #include <cstring>
 
+RT_OFFLOAD_VAR_GROUP_BEGIN
 /// Value used for asyncId when no specific stream is specified.
 static constexpr std::int64_t kNoAsyncId = -1;
+RT_OFFLOAD_VAR_GROUP_END
 
 namespace Fortran::runtime {
 

flang-rt/lib/runtime/descriptor.cpp

@@ -15,6 +15,7 @@
 #include "flang-rt/runtime/terminator.h"
 #include "flang-rt/runtime/type-info.h"
 #include "flang/Common/type-kinds.h"
+#include "flang/Runtime/freestanding-tools.h"
 #include <cassert>
 #include <cstdlib>
 #include <cstring>
@@ -26,7 +27,7 @@ RT_OFFLOAD_API_GROUP_BEGIN
 RT_API_ATTRS Descriptor::Descriptor(const Descriptor &that) { *this = that; }
 
 RT_API_ATTRS Descriptor &Descriptor::operator=(const Descriptor &that) {
-  std::memcpy(this, &that, that.SizeInBytes());
+  Fortran::runtime::memcpy(this, &that, that.SizeInBytes());
   return *this;
 }
 

flang-rt/lib/runtime/main.cpp

@@ -28,9 +28,9 @@ static void ConfigureFloatingPoint() {
 }
 
 std::thread::id _main_thread_id = std::this_thread::get_id();
-extern "C" {
 std::thread::id RTNAME(GetMainThreadId)() { return _main_thread_id; }
 
+extern "C" {
 void RTNAME(ProgramStart)(int argc, const char *argv[], const char *envp[],
     const EnvironmentDefaultList *envDefaults) {
   std::atexit(Fortran::runtime::NotifyOtherImagesOfNormalEnd);

flang/include/flang/Runtime/freestanding-tools.h

@@ -186,6 +186,7 @@ using std::memmove;
 using MemmoveFct = void *(*)(void *, const void *, std::size_t);
 
 #ifdef RT_DEVICE_COMPILATION
+[[maybe_unused]]
 static RT_API_ATTRS void *MemmoveWrapper(
     void *dest, const void *src, std::size_t count) {
   return Fortran::runtime::memmove(dest, src, count);

flang/include/flang/Runtime/main.h

@@ -15,8 +15,9 @@
 
 struct EnvironmentDefaultList;
 
-FORTRAN_EXTERN_C_BEGIN
 std::thread::id RTNAME(GetMainThreadId)();
+
+FORTRAN_EXTERN_C_BEGIN
 void RTNAME(ProgramStart)(
     int, const char *[], const char *[], const struct EnvironmentDefaultList *);
 void RTNAME(ByteswapOption)(void); // -byteswapio