[CompilerType] Remove an unused function.

Summary:
This simplifies the interface, as I'm trying to understand how
we can upstream swift support.

<rdar://problem/36377967>

Reviewers: teemperor, JDevlieghere, xiaobai, compnerd, friss

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D65781

llvm-svn: 367946