[Support] Support building LLVM for Fuchsia

[phosek]

Summary

These are necessary changes to support building LLVM for Fuchsia.
While these are not sufficient to run LLVM on Fuchsia, they are still
useful when cross-compiling LLVM libraries and runtimes for Fuchsia.

[mcgrathr]

lgtm with a comment

llvm/lib/Support/Unix/Path.inc

368

Comment something like "Fuchsia doesn't support remote filesystem mounts so far."

llvm/cmake/modules/HandleLLVMOptions.cmake

if(WIN32)
   if(CYGWIN)
     set(LLVM_ON_WIN32 0)
     set(LLVM_ON_UNIX 1)
   else(CYGWIN)
     set(LLVM_ON_WIN32 1)
     set(LLVM_ON_UNIX 0)
   endif(CYGWIN)
 else(WIN32)
-  if(UNIX)
+  if(FUCHSIA OR UNIX)
     set(LLVM_ON_WIN32 0)
     set(LLVM_ON_UNIX 1)
     if(APPLE OR ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
       set(LLVM_HAVE_LINK_VERSION_SCRIPT 0)
     else()
       set(LLVM_HAVE_LINK_VERSION_SCRIPT 1)
     endif()
-  else(UNIX)
+  else(FUCHSIA OR UNIX)
     MESSAGE(SEND_ERROR "Unable to determine platform")
-  endif(UNIX)
+  endif(FUCHSIA OR UNIX)
 endif(WIN32)
 
 set(EXEEXT ${CMAKE_EXECUTABLE_SUFFIX})
 set(LTDL_SHLIB_EXT ${CMAKE_SHARED_LIBRARY_SUFFIX})
 
 # We use *.dylib rather than *.so on darwin.
 set(LLVM_PLUGIN_EXT ${CMAKE_SHARED_LIBRARY_SUFFIX})
 

llvm/lib/Support/Unix/Path.inc

#endif
   case CIFS_MAGIC_NUMBER:
     return false;
   default:
     return true;
   }
 #elif defined(__CYGWIN__)
   // Cygwin doesn't expose this information; would need to use Win32 API.
   return false;
+#elif defined(__Fuchsia__)
+  return true;
 #elif defined(__sun)
   // statvfs::f_basetype contains a null-terminated FSType name of the mounted target
   StringRef fstype(Vfs.f_basetype);
   // NFS is the only non-local fstype??
   return !fstype.equals("nfs");
 #else
   return !!(STATVFS_F_FLAG(Vfs) & MNT_LOCAL);
 #endif