[PDB] Make variables.test to pass using a 32-bit compiler too

Summary:
`variables.test` depends on mangled names, but the mangling depends
on the bitness. This patch specifies the bitness explicitly, so mangled names
doesn't differ when a 32-bit version of the compiler is used.

Reviewers: stella.stamenova, asmith

Reviewed By: stella.stamenova

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D51158

llvm-svn: 340597