[Driver] Obey computed sysroot when finding libc++ headers.

[danalbert]

Summary

A handful of targets will try some default paths if --sysroot is not provided.
If that is the case, it should be used for the libc++ header paths.

[danalbert]

Ping?

[danalbert]

Ping.

[danalbert]

Ping.

lib/Driver/ToolChains/Linux.cpp

if (VersionText[0] == 'v' &&
         MaxVersionString = VersionText;
       }
     }
   }
   return MaxVersion ? (base + "/" + MaxVersionString).str() : "";
 }
 
 std::string Linux::findLibCxxIncludePath() const {
+  const std::string& SysRoot = computeSysRoot();
   const std::string LibCXXIncludePathCandidates[] = {
       DetectLibcxxIncludePath(getDriver().Dir + "/../include/c++"),
       // If this is a development, non-installed, clang, libcxx will
       // not be found at ../include/c++ but it likely to be found at
       // one of the following two locations:
-      DetectLibcxxIncludePath(getDriver().SysRoot + "/usr/local/include/c++"),
-      DetectLibcxxIncludePath(getDriver().SysRoot + "/usr/include/c++") };
+      DetectLibcxxIncludePath(SysRoot + "/usr/local/include/c++"),
+      DetectLibcxxIncludePath(SysRoot + "/usr/include/c++") };
   for (const auto &IncludePath : LibCXXIncludePathCandidates) {
     if (IncludePath.empty() || !getVFS().exists(IncludePath))
       continue;
     // Use the first candidate that exists.
     return IncludePath;
   }
   return "";
 }