[Driver] Use VFS to perform all distribution checks

[mgorny]

Summary

Use the VFS provided by D.getVFS() for all distribution checks,
including those performing read of the release file. Requested
by @bruno on D24954.

[bruno]

Thanks! LGTM!

cfe/trunk/lib/Driver/ToolChains.cpp

 }
 
 static bool IsUbuntu(enum Distro Distro) {
   return Distro >= UbuntuHardy && Distro <= UbuntuYakkety;
 }
 
 static Distro DetectDistro(const Driver &D, llvm::Triple::ArchType Arch) {
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> File =
-      llvm::MemoryBuffer::getFile("/etc/lsb-release");
+      D.getVFS().getBufferForFile("/etc/lsb-release");
   if (File) {
     StringRef Data = File.get()->getBuffer();
     SmallVector<StringRef, 16> Lines;
     Data.split(Lines, "\n");
     Distro Version = UnknownDistro;
     for (StringRef Line : Lines)
       if (Version == UnknownDistro && Line.startswith("DISTRIB_CODENAME="))
         Version = llvm::StringSwitch<Distro>(Line.substr(17))
for (StringRef Line : Lines)
                       .Case("wily", UbuntuWily)
                       .Case("xenial", UbuntuXenial)
                       .Case("yakkety", UbuntuYakkety)
                       .Default(UnknownDistro);
     if (Version != UnknownDistro)
       return Version;
   }
 
-  File = llvm::MemoryBuffer::getFile("/etc/redhat-release");
+  File = D.getVFS().getBufferForFile("/etc/redhat-release");
   if (File) {
     StringRef Data = File.get()->getBuffer();
     if (Data.startswith("Fedora release"))
       return Fedora;
     if (Data.startswith("Red Hat Enterprise Linux") ||
         Data.startswith("CentOS") ||
         Data.startswith("Scientific Linux")) {
       if (Data.find("release 7") != StringRef::npos)
         return RHEL7;
       else if (Data.find("release 6") != StringRef::npos)
         return RHEL6;
       else if (Data.find("release 5") != StringRef::npos)
         return RHEL5;
     }
     return UnknownDistro;
   }
 
-  File = llvm::MemoryBuffer::getFile("/etc/debian_version");
+  File = D.getVFS().getBufferForFile("/etc/debian_version");
   if (File) {
     StringRef Data = File.get()->getBuffer();
     if (Data[0] == '5')
       return DebianLenny;
     else if (Data.startswith("squeeze/sid") || Data[0] == '6')
       return DebianSqueeze;
     else if (Data.startswith("wheezy/sid") || Data[0] == '7')
       return DebianWheezy;