[LLDB][ClangExpression] Fix initialization of static enum alias members

`IntegerLiteral::Create` operates on integer types. For that reason
when we parse DWARF into an AST, when we encounter a constant
initialized enum member variable, we try to determine the underlying
integer type before creating the `IntegerLiteral`. However, we
currently don't desugar the type and for enum typedefs
`dyn_cast<EnumType>` fails. In debug builds this triggers following
assert:

```
Assertion failed: (type->isIntegerType() && "Illegal type in IntegerLiteral"), function IntegerLiteral, file Expr.cpp, line 892
```

This patch turns the `dyn_cast<EnumType>` into a `getAs<EnumType>`
which `dyn_cast`s the canonical type, allowing us to get to the
underlying integer type.

**Testing**

* API test
* Manual repro is fixed

Differential Revision: https://reviews.llvm.org/D130213