[clang] Silence warning when building with MSVC targetting x86

This fixes:
```
[3786/6996] Building CXX object tools\clang\lib\AST\CMakeFiles\obj.clangAST.dir\AttrDocTable.cpp.obj
C:\git\llvm-project\clang\lib\AST\AttrDocTable.cpp(24): warning C4018: '<': signed/unsigned mismatch
```

clang/lib/AST/AttrDocTable.cpp

@@ -21,7 +21,7 @@ static const llvm::StringRef AttrDoc[] = {
 };
 
 llvm::StringRef clang::Attr::getDocumentation(clang::attr::Kind K) {
-  if (K < std::size(AttrDoc))
+  if (K < (int)std::size(AttrDoc))
     return AttrDoc[K];
   return "";
 }