[lldb][Language] List supported languages in expr error text

Before:
```
(lldb) expr --language abc -- 1 + 1
error: unknown language type: 'abc' for expression
```

After:
```
(lldb) expr --language abc -- 1 + 1
error: unknown language type: 'abc' for expression. List of supported languages:
  c++
  objective-c++
  c++03
  c++11
  c++14
  objc++
```

We choose to only list the languages which `expr` will actually
accept instead of all the language constants defined in `Language.cpp`
since that's what the user will most likely need.

Differential Revision: https://reviews.llvm.org/D142034