Driver: reuse getCompilerRT in place of addSanitizerRTWindows

The logic for addSanitizerRTWindows was performing the same logical operation as
getCompilerRT, which was previously fully generalised for Linux and Windows.
This avoids having a duplication of the logic for building up the name of a
clang_rt component.  This change does move the current limitation for Windows
into getArchNameForCompilerRTLib, where it is assumed that the architecture for
Windows is always i386.

llvm-svn: 225087