A couple of minor build fixes for Windows Arm host

[arm-chrjan01]

Summary

Use the Arm version of the Debug Interface Access SDK when cross-compiling for an Arm host using the Visual Studio compiler.
Use CONTEXT.R11 instead of CONTEXT.Fp. Fp field does not exist in Windows SDK 10.0.16299.0, at least.

[arm-chrjan01]

Ping for review.

lib/DebugInfo/PDB/CMakeLists.txt

 macro(add_pdb_impl_folder group)
   list(APPEND PDB_IMPL_SOURCES ${ARGN})
   source_group(${group} FILES ${ARGN})
 endmacro()
 
 if(LLVM_ENABLE_DIA_SDK)
   include_directories(${MSVC_DIA_SDK_DIR}/include)
   set(LIBPDB_LINK_FOLDERS "${MSVC_DIA_SDK_DIR}\\lib")
-  if (CMAKE_SIZEOF_VOID_P EQUAL 8)
+  if ("$ENV{VSCMD_ARG_TGT_ARCH}" STREQUAL "arm64")
+    set(LIBPDB_LINK_FOLDERS "${LIBPDB_LINK_FOLDERS}\\arm64")
+  elseif ("$ENV{VSCMD_ARG_TGT_ARCH}" STREQUAL "arm")
+    set(LIBPDB_LINK_FOLDERS "${LIBPDB_LINK_FOLDERS}\\arm")
+  elseif (CMAKE_SIZEOF_VOID_P EQUAL 8)
     set(LIBPDB_LINK_FOLDERS "${LIBPDB_LINK_FOLDERS}\\amd64")
   endif()
   file(TO_CMAKE_PATH "${LIBPDB_LINK_FOLDERS}\\diaguids.lib" LIBPDB_ADDITIONAL_LIBRARIES)
 
   add_pdb_impl_folder(DIA
     DIA/DIADataStream.cpp
     DIA/DIAEnumDebugStreams.cpp
     DIA/DIAEnumInjectedSources.cpp

lib/Support/Windows/Signals.inc

#if defined(_M_X64)
   StackFrame.AddrFrame.Offset = Context.Rbp;
 #elif defined(_M_IX86)
   StackFrame.AddrPC.Offset = Context.Eip;
   StackFrame.AddrStack.Offset = Context.Esp;
   StackFrame.AddrFrame.Offset = Context.Ebp;
 #elif defined(_M_ARM64) || defined(_M_ARM)
   StackFrame.AddrPC.Offset = Context.Pc;
   StackFrame.AddrStack.Offset = Context.Sp;
-  StackFrame.AddrFrame.Offset = Context.Fp;
+  StackFrame.AddrFrame.Offset = Context.R11;
 #endif
   StackFrame.AddrPC.Mode = AddrModeFlat;
   StackFrame.AddrStack.Mode = AddrModeFlat;
   StackFrame.AddrFrame.Mode = AddrModeFlat;
   PrintStackTraceForThread(OS, GetCurrentProcess(), GetCurrentThread(),
                            StackFrame, &Context);
 }
 
#elif defined(_M_IX86)
   StackFrame.AddrStack.Mode = AddrModeFlat;
   StackFrame.AddrFrame.Offset = ep->ContextRecord->Ebp;
   StackFrame.AddrFrame.Mode = AddrModeFlat;
 #elif defined(_M_ARM64) || defined(_M_ARM)
   StackFrame.AddrPC.Offset = ep->ContextRecord->Pc;
   StackFrame.AddrPC.Mode = AddrModeFlat;
   StackFrame.AddrStack.Offset = ep->ContextRecord->Sp;
   StackFrame.AddrStack.Mode = AddrModeFlat;
-  StackFrame.AddrFrame.Offset = ep->ContextRecord->Fp;
+  StackFrame.AddrFrame.Offset = ep->ContextRecord->R11;
   StackFrame.AddrFrame.Mode = AddrModeFlat;
 #endif
 
   HANDLE hProcess = GetCurrentProcess();
   HANDLE hThread = GetCurrentThread();
   PrintStackTraceForThread(llvm::errs(), hProcess, hThread, StackFrame,
                            ep->ContextRecord);