Reland "Move the test compiler setup in a common place. NFCI"

Original commit message: "
Move the test compiler setup in a common place. NFCI

This patch reduces the copy paste in the unittest/CodeGen folder by moving the
common compiler setup phase in a header file.

Differential revision: https://reviews.llvm.org/D91061
"

This patch includes a fix for the memory leaks pointed out by @vitalybuka