[lldbsuite] Invoke sed on Windows to determine the cache dir for clang

Summary: In order to invoke sed on Windows, we need to quote the command correctly. Since we already have commands which do that, move the definitions at the beginning of the file and then re-use them for each command.

Reviewers: aprantl, zturner

Subscribers: teemperor, lldb-commits

Differential Revision: https://reviews.llvm.org/D54709

llvm-svn: 347243