[clang] [clang-tools-extra] [WIP] [clang][DebugInfo] Use mangling number and scope to create unambiguous names for anonymous structures (PR #168533)

[Michael Buch via cfe-commits]

Michael137 wrote:

@dwblaikie @rnk It's been a minute but here's the "canonical lambda names in debug-info" that we talked about in https://github.com/llvm/llvm-project/pull/159592

The interesting bit is the 3rd commit.

Before:
```
  // CHECK: !DISubprogram(name: "f1<(lambda at {{.*}}simple-template-names.cpp:[[# @LINE - 2]]:17)>",
```
After:
```
  // CHECK: !DISubprogram(name: "f1<f1()::(lambda1)>",
```

Still in draft mode until I verify that this does the right thing for namespaces too.

https://github.com/llvm/llvm-project/pull/168533