[clang-tidy] Fix `llvmlibc-inline-function-decl` false positives for templated function definitions

For a declaration the `FunctionDecl` begin location does not include the
template parameter lists, but for some reason if you have a separate
definitions to the declaration the begin location does include them.
With this patch we now correctly handle that case.

This fixes llvm#62746

Reviewed By: PiotrZSL

Differential Revision: https://reviews.llvm.org/D153218