This change adds LLVM intrinsics and clang builtins for the `cvt`
RS rounding mode instruction variants.

Tests are added in `convert-sm103a.ll` and verified through ptxas-13.0.

### Summary
Reland: wcscpy/wcsncpy interceptors and stabilize wchar tests on
Darwin/Android. Functional reland (runtime + tests).

### Context
Reland of #160493 and #161624; previously reverted by #162021 and
#162001 to restore green.

### Motivation
- Restore wchar interceptors (wcscpy/wcsncpy), broaden ASan coverage,
and improve Windows parity with narrow-string checks.
- Make tests robust across Darwin/Android to keep bots green.

### Runtime (wcscpy/wcsncpy)
- Add overlap checks; mark read/write ranges in bytes.
- Use MaybeRealWcsnlen when available to bound reads.
- Register Windows static runtime thunk where applicable.

### Tests (wcscpy/wcsncpy/wcscat/wcsncat)
- Android: keep `%env_asan_opts=log_to_stderr=1` so the ASan header is
on stderr.
- Darwin: tolerate reordering by putting all four key lines in one DAG
group:

```cpp
// CHECK-DAG: Good so far.
// CHECK-DAG: ERROR: AddressSanitizer: stack-buffer-overflow on address [[ADDR:...]] at pc {{...}} bp {{...}} sp {{...}}
// CHECK-DAG: WRITE of size {{[0-9]+}} at [[ADDR]] thread T0
// CHECK-DAG: #0 {{0x[0-9a-f]+}} in <func>
```

### Risk
- Functional reland (runtime + tests), intended to restore functionality
and maintain stability across platforms.

---------

Signed-off-by: Yixuan Cao <caoyixuan2019@email.szu.edu.cn>

Reapply llvm/llvm-project#157204 with fix and a new test for the issue
it caused (the test change provoked the assert that was converted to an
if condition).

Also, make the application of this new attribute under an (on by
default) flag, so that it can be more easily disabled if needed. Add
test for the new flag.

The redundant notes were introduced with the workaround for finding the
template instantiationa args for lambdas inside template type aliases.

This removes the notes for the cases where we are simply instantiating
an outer template, and when diagnosing uses of the alias template.

Also adds comments calling the workaround explicitly.