[clang] Fix crash for sizeof on VLAs

Adds overload of TransformToPotentiallyEvaluated for TypeSourceInfo to
properly deal with VLAs in nested calls of sizeof and typeof. Fixes
PR31042 (https://github.com/llvm/llvm-project/issues/30390).

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D27800