Delete type_sp member from TypePair

Summary:
As discussed in the review of D59217, this member is unnecessary since
always the first thing we do is convert it to a CompilerType.

This opens up possibilities for further cleanups (e.g. the whole
TypePair class now loses purpose, since we can just pass around
CompilerType everywhere), but I did not want to do that yet, because I
am not sure if this will not introduce breakages in some of the
platforms/configurations that I am not testing on.

Reviewers: clayborg, zturner, jingham

Subscribers: jdoerfert, lldb-commits

Differential Revision: https://reviews.llvm.org/D59297

llvm-svn: 356262