[clang][DebugInfo] Emit DW_AT_type of preferred name if available

With this patch, whenever we emit a `DW_AT_type` for some declaration
and the type is a template class with a `clang::PreferredNameAttr`, we
will emit the typedef that the attribute refers to instead. I.e.,

```
0x123 DW_TAG_variable
        DW_AT_name "var"
        DW_AT_type (0x123 "basic_string<char>")

0x124 DW_TAG_structure_type
        DW_AT_name "basic_string<char>"
```
...becomes
```
0x123 DW_TAG_variable
        DW_AT_name "var"
        DW_AT_type (0x124 "std::string")

0x124 DW_TAG_structure_type
        DW_AT_name "basic_string<char>"

0x125 DW_TAG_typedef
        DW_AT_name "std::string"
        DW_AT_type (0x124 "basic_string<char>")
```

We do this by returning the preferred name typedef `DIType` when
we create a structure definition. In some cases, e.g., with `-gmodules`,
we don't complete the structure definition immediately but do so later
via `completeClassData`, which overwrites the `TypeCache`. In such cases
we don't actually want to rewrite the cache with the preferred name. We
handle this by returning both the definition and the preferred typedef
from `CreateTypeDefinition` and let the callee decide what to do with
it.

Essentially we set up the types as:
```
TypeCache[Record] => DICompositeType
ReplaceMap[Record] => DIDerivedType(baseType: DICompositeType)
```

For now we keep this behind LLDB tuning.

**Testing**

- Added clang unit-test
- `check-llvm`, `check-clang` pass
- Confirmed that this change correctly repoints
  `basic_string` references in some of my test programs.
- Will add follow-up LLDB API tests

Differential Revision: https://reviews.llvm.org/D145803