Work around an internal compiler error in MSVC.

For some reason MSVC ICEs when trying to index into a map using
a temporary object.  Work around this by separating out the call
into multiple lines.

Patch by Aidan Dodds
Differential Revision: http://reviews.llvm.org/D6702
Reviewed by: Zachary Turner, Greg Clayton

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@224443 91177308-0d34-0410-b5e6-96231b3b80d8