Auto merge of #51090 - kennytm:tidy-check-missing-tracking-issue, r=alexcrichton

Ensure every unstable language feature has a tracking issue.

Filled in the missing numbers:

* `abi_ptx` → #38788
* `generators` → #43122
* `global_allocator` → #27389

Reused existing tracking issues because they were decomposed from a larger feature

* `*_target_feature` → #44839 (reusing the old `target_feature` number)
* `proc_macros_*` → #38356 (reusing the to-be-stabilized `proc_macros` number)

Filed new issues

* `exhaustive_patterns` → #51085
* `pattern_parentheses` → #51087
* `wasm_custom_section` and `wasm_import_module` → #51088