[clang] [clang-tools-extra] [libcxx] [clang] improved preservation of template keyword (PR #133610)

[Devon Loehr via cfe-commits]

DKLoehr wrote:

We're also seeing `class member cannot be redeclared` errors when trying to build chromium:
```
In file included from ../../components/system_cpu/cpu_probe.cc:16:
In file included from ../../components/system_cpu/cpu_probe_linux.h:14:
../../base/threading/sequence_bound.h:281:8: error: class member cannot be redeclared
  281 |   void PostTaskWithThisObject(
      |        ^
../../base/threading/sequence_bound.h:264:8: note: previous definition is here
  264 |   void PostTaskWithThisObject(
      |        ^
```
The file in question can be viewed [here](https://source.chromium.org/chromium/chromium/src/+/main:base/threading/sequence_bound.h;l=263;drc=72ba73a81b090ee221a9f107333c3e5e964cf750). It seems like the compiler is getting confused between `const UnwrappedT&` and `UnwrappedT*`, where `UnwrappedT` is, ultimately, a templated type. I'm also looking for a reduced repro, though I expect alexfh will produce one that works for both of us.

https://github.com/llvm/llvm-project/pull/133610