Reset the system compiler each time we start a new flavour. Otherwise

Reset the system compiler each time we start a new flavour.  Otherwise
the last compiler built for the previous flavour is used for the next,
for example the Debug clang compiler was being used for the initial build
of the Release LLVM.  Flavors should be independent of each other.  This
especially matters if the compiler built for the previous flavour doesn't
actually work!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142607 91177308-0d34-0410-b5e6-96231b3b80d8