[Tooling/Inclusion] Avoid narrowing conversions in macro expansion (#72664)

```
clang/lib/Tooling/Inclusions/Stdlib/StandardLibrary.cpp:144:65: warning:
 narrowing conversion of ‘llvm::StringRef(((const char*)"std::experiment
al::filesystem::")).llvm::StringRef::size()’ from ‘size_t’ {aka ‘long un
signed int’} to ‘unsigned int’ [-Wnarrowing]
  144 | #define SYMBOL(Name, NS, Header) {#NS #Name, StringRef(#NS).size
(), #Header},
      |                                              ~~~~~~~~~~~~~~~~~~~
^~
clang/lib/Tooling/Inclusions/Stdlib/StdTsSymbolMap.inc:51:1: note: in ex
pansion of macro ‘SYMBOL’
   51 | SYMBOL(temp_directory_path, std::experimental::filesystem::, <ex
perimental/filesystem>)
      | ^~~~~~
```