[Support] Use C++11 attribute syntax for visibility attributes

[tstellar]

Summary

The gnu extension __attribute syntax cannot be mixed with the
C++11 alignas specifier, so in order to use visibility attributes on
classes that also use alignas, we need to use the C++11 standard syntax.

Also fix a few warnings introduced by this change.

[compnerd]

Thanks for fixing the attribution in the various cases. It is unfortunate that we cannot use the GNU __attribute__ spelling. I suppose that someday we will be able to use the C23 attributes and get a single spelling again.

llvm/include/llvm/Support/Compiler.h

 /// into a shared library, then the class should be private to the library and
 /// not accessible from outside it.  Can also be used to mark variables and
 /// functions, making them private to any shared library they are linked into.
 /// On PE/COFF targets, library visibility is the default, so this isn't needed.
 ///
 /// LLVM_EXTERNAL_VISIBILITY - classes, functions, and variables marked with
 /// this attribute will be made public and visible outside of any shared library
 /// they are linked in to.
-#if __has_attribute(visibility) &&                                             \
-    (!(defined(_WIN32) || defined(__CYGWIN__)) ||                              \
+
+#if LLVM_HAS_CPP_ATTRIBUTE(gnu::visibility)
+#define LLVM_ATTRIBUTE_VISIBILITY_HIDDEN [[gnu::visibility("hidden")]]
+#define LLVM_ATTRIBUTE_VISIBILITY_DEFAULT [[gnu::visibility("default")]]
+#elif __has_attribute(visibility)
+#define LLVM_ATTRIBUTE_VISIBILITY_HIDDEN __attribute__((visibility("hidden")))
+#define LLVM_ATTRIBUTE_VISIBILITY_DEFAULT __attribute__((visibility("default")))
+#else
+#define LLVM_ATTRIBUTE_VISIBILITY_HIDDEN
+#define LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
+#endif
+
+
+#if (!(defined(_WIN32) || defined(__CYGWIN__)) ||                              \
      (defined(__MINGW32__) && defined(__clang__)))
-#define LLVM_LIBRARY_VISIBILITY __attribute__ ((visibility("hidden")))
+#define LLVM_LIBRARY_VISIBILITY LLVM_ATTRIBUTE_VISIBILITY_HIDDEN
 #if defined(LLVM_BUILD_LLVM_DYLIB) || defined(LLVM_BUILD_SHARED_LIBS)
-#define LLVM_EXTERNAL_VISIBILITY __attribute__((visibility("default")))
+#define LLVM_EXTERNAL_VISIBILITY LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #else
 #define LLVM_EXTERNAL_VISIBILITY
 #endif
 #else
 #define LLVM_LIBRARY_VISIBILITY
 #define LLVM_EXTERNAL_VISIBILITY
 #endif
 

llvm/include/llvm/Transforms/Scalar/GVN.h

 class MemorySSAUpdater;
 class NonLocalDepResult;
 class OptimizationRemarkEmitter;
 class PHINode;
 class TargetLibraryInfo;
 class Value;
 /// A private "module" namespace for types and utilities used by GVN. These
 /// are implementation details and should not be used by clients.
-namespace gvn LLVM_LIBRARY_VISIBILITY {
+namespace LLVM_LIBRARY_VISIBILITY gvn {
 
 struct AvailableValue;
 struct AvailableValueInBlock;
 class GVNLegacyPass;
 
 } // end namespace gvn
 
 /// A set of parameters to control various transforms performed by GVN pass.

llvm/include/llvm/Transforms/Scalar/SROA.h

 class Function;
 class LLVMContext;
 class PHINode;
 class SelectInst;
 class Use;
 
 /// A private "module" namespace for types and utilities used by SROA. These
 /// are implementation details and should not be used by clients.
-namespace sroa LLVM_LIBRARY_VISIBILITY {
+namespace LLVM_LIBRARY_VISIBILITY sroa {
 
 class AllocaSliceRewriter;
 class AllocaSlices;
 class Partition;
 class SROALegacyPass;
 
 class SelectHandSpeculativity {
   unsigned char Storage = 0; // None are speculatable by default.

llvm/lib/Target/AMDGPU/AMDGPUAsmPrinter.cpp

 }
 
 static AsmPrinter *
 createAMDGPUAsmPrinterPass(TargetMachine &tm,
                            std::unique_ptr<MCStreamer> &&Streamer) {
   return new AMDGPUAsmPrinter(tm, std::move(Streamer));
 }
 
-extern "C" void LLVM_EXTERNAL_VISIBILITY LLVMInitializeAMDGPUAsmPrinter() {
+extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeAMDGPUAsmPrinter() {
   TargetRegistry::RegisterAsmPrinter(getTheR600Target(),
                                      llvm::createR600AsmPrinterPass);
   TargetRegistry::RegisterAsmPrinter(getTheGCNTarget(),
                                      createAMDGPUAsmPrinterPass);
 }
 
 AMDGPUAsmPrinter::AMDGPUAsmPrinter(TargetMachine &TM,
                                    std::unique_ptr<MCStreamer> Streamer)