[clang][Sema] Fix initialization of `NonTypeTemplateParmDecl`... (#121768)

...when there are invalid constraints.

When attaching a `TypeConstraint`, in case of error, the trailing
pointer that is supposed to point to the constraint is left
uninitialized.
Sometimes the uninitialized value will be a `nullptr`, but at other
times it will not. If we traverse the AST (for instance, dumping it, or
when writing the BMI), we may get a crash depending on the value that
was left. The serialization may also contain a bogus value.

In this commit, we always initialize the `PlaceholderTypeConstraint`
with `nullptr`, to avoid accessing this uninitialized memory.

This does not affect only modules, but it causes a segfault more
consistently when they are involved.

The test case was reduced from `mp-units`.

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Co-authored-by: Erich Keane <ekeane@nvidia.com>

0 files changed, 0 insertions, 0 deletions