[clang codegen] Fix MS ABI detection of user-provided constructors. (l…

…lvm#90151)

In the context of determining whether a class counts as an "aggregate",
a constructor template counts as a user-provided constructor.

Fixes llvm#86384

(cherry picked from commit 3ab4ae9)

clang/docs/ReleaseNotes.rst

@@ -79,6 +79,12 @@ ABI Changes in This Version
 
 - Fixed Microsoft name mangling of placeholder, auto and decltype(auto), return types for MSVC 1920+. This change resolves incompatibilities with code compiled by MSVC 1920+ but will introduce incompatibilities with code compiled by earlier versions of Clang unless such code is built with the compiler option -fms-compatibility-version=19.14 to imitate the MSVC 1914 mangling behavior.
 
+- Fixed Microsoft calling convention for returning certain classes with a
+  templated constructor. If a class has a templated constructor, it should
+  be returned indirectly even if it meets all the other requirements for
+  returning a class in a register. This affects some uses of std::pair.
+  (#GH86384).
+
 AST Dumping Potentially Breaking Changes
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