[Clang] Avoid misleading 'conflicting types' diagnostic with no-prototype decls.

Clang has recently started diagnosing prototype redeclaration errors like [rG385e7df33046](https://reviews.llvm.org/rG385e7df33046d7292612ee1e3ac00a59d8bc0441)

This flagged legitimate issues in a codebase but was confusing to resolve because it actually conflicted with a function declaration from a system header and not from the one emitted with "note: ".

This patch updates the error handling to use the canonical declaration's source location instead to avoid misleading errors like the one described.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D126258