Enable -Wunused-template by default (add to -Wall)

[flash1729]

-Wunused-template already exists in Clang, but it's DefaultIgnore and commented out of the Unused diagnostic group in clang/include/clang/Basic/DiagnosticGroups.td, so it isn't part of -Wall. This issue tracks cleaning up the remaining in-tree occurrences and then enabling the warning.

Why it helps

A static (or anonymous-namespace) function template defined in a header has internal linkage, so every translation unit that includes the header gets its own copy. When such a template is referenced from another inline function or template in a header, the definitions across TUs refer to different entities, which is a latent ODR violation (ill-formed, no diagnostic required). -Wunused-template surfaces these, and enabling it keeps the pattern from coming back.

Status

Building LLVM + Clang with the flag enabled:

• Started at 652 raw warnings (46 unique sites) across ~24 files.
• After the cleanup below, the tree builds completely clean (0 warnings).
• libc++/libc++abi/libunwind already build clean (handled in D144667), so the historical "clean up libc++ first" note is stale. The remaining work is in LLVM core and Clang.

Approach

Three kinds of warning, each with its own fix:

1. static or anonymous-namespace function template in a header: remove the internal linkage (drop static, or move it out of the anonymous namespace into a named one). Templates are implicitly inline, so nothing is added. This is the ODR fix.
2. Template in a .cpp that's never instantiated: dead code, delete it.
3. Template used only inside assert() or LLVM_DEBUG() (compiled out in release): mark [[maybe_unused]].

Cleanup PRs (per area, all NFC)

• [Object][ELF] Remove internal linkage from header function templates (NFC) / Approved, Wait for past contributor to acknowledge the change
• [Mips] Remove unused function templates (NFC) / Approved
• [JITLink][ORC] Clean up unused and assert-only function templates (NFC) / Approved
• [VPlan] Remove internal linkage from findUserOf templates (NFC) / Approved need to merge
• [IR] Remove unused and mark debug-only verifier templates (NFC)
• [clang] Fix -Wunused-template in AST/analysis helpers (NFC)
• [clang] Clean up unused/assert-only frontend helpers (NFC)
• [FunctionAttrs] Remove unused legacy-PM runImpl template (NFC)
• [OpenMP] Remove unused isStrictSubset template (NFC)
• Assorted library helpers — Support / Bitcode / Coverage / ProfileData / MC / llvm-rc (each NFC) / Approved

Final step

• [Clang] Enable -Wunused-template under -Wall -- uncomment UnusedTemplate in DiagnosticGroups.td and update tests

Prior art

• D29877: original implementation of -Wunused-template (2017).
• D144667: [libc++] Enable -Wunused-template (landed).
• D143524: earlier attempt to enable it by default (didn't land, bandwidth).