Hide stderr output from lldb-argdumper

Under very specific circumstances the default shell /bin/sh might
print stuff to stderr before launching lldb-argdumper, which then
confuses the JSON parser. This patch suppresses stderr output from
lldb-argdumper to avoid this situation.

rdar://problem/50149390

Differential Revision: https://reviews.llvm.org/D61101

llvm-svn: 359156