This patch removes some files made obsolete by newer version
of them available in the Optimizer directory.
flang/include/flang/Lower/FIRBuilder.h and flang/lib/Lower/FIRBuilder.cpp are
removed and replace by the newer version present in
flang/include/flang/Optimizer/Builder/FIRBuilder.h and
flang/lib/Optimizer/Builder/FIRBuilder.cpp.
flang/include/flang/Lower/Support/BoxValue.h and flang/lib/Lower/ConvertExpr.cpp are removed and replace by the newer
version present in flang/include/flang/Optimizer/Builder/BoxValue.h
This patch is a preparation to be able to upstream the lowering from
fir-dev.