[OPENMP]Fix PR46593: Reduction initializer missing construnctor call.

Summary:
If user-defined reductions with the initializer are used with classes,
the compiler misses the constructor call when trying to create a private
copy of the reduction variable.

Reviewers: jdoerfert

Subscribers: cfe-commits, yaxunl, guansong, caomhin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83334

(cherry picked from commit 41d0af00)