[lldb] add a missing dependency on intrinsics_gen

Sometimes builds will fail with errors like:

```
In file included from /build/external/llvm-project/lldb/source/Symbol/SwiftASTContext.cpp:52:
In file included from /build/external/swift/include/swift/IRGen/Linking.h:22:
In file included from /build/external/swift/include/swift/SIL/SILFunction.h:24:
In file included from /build/external/swift/include/swift/SIL/SILBasicBlock.h:23:
In file included from /build/external/swift/include/swift/SIL/SILInstruction.h:21:
In file included from /build/external/swift/include/swift/AST/Builtins.h:24:
**/build/external/llvm-project/llvm/include/llvm/IR/Attributes.h:74:14: fatal error: 'llvm/IR/Attributes.inc' file not found**
**^~~~~~~~~~~~~~~~~~~~~~~~**
```
This change ensures the `Attributes.inc` file is generated before building `SwiftASTContext.cpp`.

Differential Revision: https://reviews.llvm.org/D90857