[NativePDB] Fix ast-reconstruction test on x86

Summary:
This patch fixes ast-reconstruction.cpp test on x86 platform.

Patch by: leonid.mashinskiy

Reviewers: zturner, stella.stamenova

Reviewed By: zturner

Subscribers: aleksandr.urakov, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D55002

llvm-svn: 347975