reduce mangled name sizes via link-time symbol renaming

[Timothee Cour]

could a solution like proposed below be adapted to automatically
reduce size of long symbol names?

It allows final object files to be smaller; eg see the problem this causes:

* String Switch Lowering:
http://forum.dlang.org/thread/p4d777$1vij$1@digitalmars.com
caution: NSFW! contains huge mangled symbol name!
* http://lists.llvm.org/pipermail/lldb-dev/2018-January/013180.html
"[lldb-dev] Huge mangled names are causing long delays when loading
symbol table symbols")


```
main.d:
void foo_test1(){ }
void main(){ foo_test1(); }

dmd -c libmain.a

ld -r libmain.a -o libmain2.a -alias _D4main9foo_test1FZv _foobar
-unexported_symbol _D4main9foo_test1FZv
# or : via `-alias_list filename`

#NOTE: dummy.d only needed because somehow dmd needs at least one
object file or source file, a static library is somehow not enough
(dmd bug?)

dmd -of=main2 libmain2.a dummy.d

nm main2 | grep _foobar # ok

./main2 # ok
```

NOTE: to automate this process it could find all symbol names >
threshold and apply a mapping form long mangled names to short aliases
(eg: object_file_name + incremented_counter), that file with all the
mappings can be supplied for a demangler (eg for lldb/gdb debugging
etc)