[LLDB][NativePDB] Use undecorated name for types if UniqueName isn't mangled (#152114)

Languages other than C/C++ don't necessarily emit mangled names in the
`UniqueName` field of type records. Rust specifically emits a unique ID
that doesn't contain the name.

For example, `(i32, i32)` is emitted as

```llvm
!266 = !DICompositeType(
  tag: DW_TAG_structure_type, name: "tuple$<i32,i32>", file: !9, size: 64, align: 32,
  elements: !267, templateParams: !17, identifier: "19122721b0632fe96c0dd37477674472"
)
```

which results in

```
  0x1091 | LF_STRUCTURE [size = 72, hash = 0x1AC67] `tuple$<i32,i32>`
           unique name: `19122721b0632fe96c0dd37477674472`
           vtable: <no type>, base list: <no type>, field list: 0x1090
           options: has unique name, sizeof 8
```

In C++ with Clang and MSVC, a structure similar to this would result in
```
   0x136F | LF_STRUCTURE [size = 44, hash = 0x30BE2] `MyTuple`
            unique name: `.?AUMyTuple@@`
            vtable: <no type>, base list: <no type>, field list: 0x136E
            options: has unique name, sizeof 8
```

With this PR, if a `UniqueName` is encountered that couldn't be parsed,
it will fall back to using the undecorated (→ do the same as if the
unique name is empty/unavailable).

I'm not sure how to test this. Maybe compiling the LLVM IR that rustc
emits?

Fixes #152051.