[clang] [OpenMP 6.0] Parse/Sema support for reduction over private variable with reduction clause. (PR #129938)

[CHANDRA GHALE via cfe-commits]

chandraghale wrote:

1. Used tail-allocated storage for extra members added in OMPreductionClause. 
2. Modified assertion to diagnostic in parsing logic. 
3. Reduction clause modifiers reduction-modifier and original-sharingmodifier packed into existing structure. 
4. Added additional lit test cases. 

https://github.com/llvm/llvm-project/pull/129938