[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