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
b4a0b9fab4