Remove LLDB_DEFAULT_SHELL #define, and determine this at runtime.

Differential Revision: http://reviews.llvm.org/D5805
Reviewed by: Greg Clayton

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@220217 91177308-0d34-0410-b5e6-96231b3b80d8
Don't use mkstemp, as it doesn't exist on Windows.

Differential Revision: http://reviews.llvm.org/D5849
Reviewed by: Jason Molenda

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@220218 91177308-0d34-0410-b5e6-96231b3b80d8