Fixed IRExecutionUnit so that it looks up addresses
correctly.  We have been getting lucky since most
expressions generate only one section (or the first
code section contains all the code), but sometimes
it actually matters.

<rdar://problem/14180236>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@185054 91177308-0d34-0410-b5e6-96231b3b80d8