[cxxmodules] Autoload less libraries

In previous allmodules&autoloading patch, we used callback from
DeserializationListener to get Decl and loaded corresponding libraries.
It worked, but the performance was bad because ROOT was loading
excessive libraries.

In this patch, we use TCling::LazyFunctionCreatorAutoloadForModule. This
function gets callback when "mangled_name" was not found in loaded
libraries thus we have to the load corresponding library and lookup
again.

I used unordered_map to store mangled identifier and library pair. I'm
doing an optimization by hashing mangled name and storing library not by
name but by uint8 and hold uint8-name information in another vector.
Also tried std::map but unorderd_map was more performant. There are
better hash table like:
https://probablydance.com/2018/05/28/a-new-fast-hash-table-in-response-to-googles-new-fast-hash-table/
we can try to use them if this part gets crucial.

With this patch:
```
Processing tutorials/hsimple.C...
hsimple   : Real Time =   0.04 seconds Cpu Time =   0.03 seconds
(TFile *) 0x562b37a14fe0
Processing /home/yuka/CERN/ROOT/memory.C...
cpu  time = 0.362307 seconds
sys  time = 0.039741 seconds
res  memory = 278.215 Mbytes
vir  memory = 448.973 Mbytes
```

W/o this patch:
```
Processing tutorials/hsimple.C...
hsimple   : Real Time =   0.08 seconds Cpu Time =   0.07 seconds
(TFile *) 0x5563018a1d30
Processing /home/yuka/CERN/ROOT/memory.C...
cpu  time = 1.524314 seconds
sys  time = 0.157075 seconds
res  memory = 546.867 Mbytes
vir  memory = 895.184 Mbytes
```

So it improves time by 4x times and memory by 2x.