Support expanding builtin packs in more context

[ilya-biryukov]

Builtin pack expansions introduced in #106730 can only be expanded in template arguments and bases right now.
This limitation should be lifted by introducing the relevant code paths. For a list of examples see

llvm-project/clang/test/SemaTemplate/dedup-types-builtin.cpp

Line 109 in db63c57

// The following cases are not supported yet, should produce an error.

For the code changes necessary, see how template arguments and bases are handled in #106730.
We might also have the opportunity to reduce the code duplication from expanding template packs in most places in the code, although this would be a stretch outcome.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Ilya Biryukov (ilya-biryukov)

Builtin pack expansions introduced in #106730 can only be expanded in template arguments and bases right now. This limitation should be lifted by introducing the relevant code paths. For a list of examples see https://github.com/llvm/llvm-project/blob/db63c57c59e67c65f487b2d88cbc9ac89dfdd01b/clang/test/SemaTemplate/dedup-types-builtin.cpp#L109

For the code changes necessary, see how template arguments and bases are handled in #106730.
We might also have the opportunity to reduce the code duplication from expanding template packs in most places in the code, although this would be a stretch outcome.