[Driver] Default to libc++ on FreeBSD

Downstream may naively translate between DSL and LLVM target
triple. If OS version is lost in the process then Clang would
default to a version that's no longer supported by OS vendor.

https://reviews.llvm.org/D77776

--- clang/lib/Driver/ToolChains/FreeBSD.cpp.orig	2019-12-11 19:15:30 UTC
+++ clang/lib/Driver/ToolChains/FreeBSD.cpp
@@ -351,7 +351,8 @@ FreeBSD::FreeBSD(const Driver &D, const llvm::Triple &
 }
 
 ToolChain::CXXStdlibType FreeBSD::GetDefaultCXXStdlibType() const {
-  if (getTriple().getOSMajorVersion() >= 10)
+  unsigned Major = getTriple().getOSMajorVersion();
+  if (Major >= 10 || Major == 0)
     return ToolChain::CST_Libcxx;
   return ToolChain::CST_Libstdcxx;
 }