Remove the TypePair class

Summary:
After D59297, the TypePair class kind of lost its purpose as it was no
longer a "pair". This finishes the job started in that patch and deletes
the class altogether. All usages have been updated to use CompilerType
class directly.

Reviewers: clayborg, jingham, zturner

Subscribers: mehdi_amini, dexonsmith, jdoerfert, lldb-commits

Differential Revision: https://reviews.llvm.org/D59414

llvm-svn: 356993