[clang-tidy] Fix false positives in readability-redundant-inline-specifier (#135391)

The out-of-line explicitly-defaulted definition is not the first
declaration, so it is not implicitly inline.

Alt. reference:
9.5.2 (3) Explicitly-defaulted functions in
[N4950](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/n4950.pdf).
or https://timsong-cpp.github.io/cppwp/n4861/dcl.fct.def.default#3

Fixes #130745

---------

Signed-off-by: Björn Svensson <bjorn.a.svensson@est.tech>

clang-tools-extra/docs/ReleaseNotes.rst

@@ -313,6 +313,10 @@ Changes in existing checks
   <clang-tidy/checks/readability/qualified-auto>` check by adding the option
   `AllowedTypes`, that excludes specified types from adding qualifiers.
 
+- Improved :doc:`readability-redundant-inline-specifier
+  <clang-tidy/checks/readability/redundant-inline-specifier>` check by fixing
+  false positives on out-of-line explicitly defaulted functions.
+
 - Improved :doc:`readability-redundant-smartptr-get
   <clang-tidy/checks/readability/redundant-smartptr-get>` check by fixing
   some false positives involving smart pointers to arrays.