Treat libclang_rt.* as normal library if the user uses --sysroot=

[trixirt]

Summary

In cross compiling, one normally assumes that the user supplied sysroot contains all of the libraries needed in the link. And so the linker adjusts the -L paths to look in the sysroot.
However because libclang_rt.* is supplied at link time as a full path, the link can not find it in the sysroot.

This change will churn the user's current experience and the unit tests.
I will take take of the tests, if folks are ok with the change.

[atanasyan]

Will it work on Windows? If sysroot is provided, Prefix is "-l", but I'm not sure that link.exe accepts "-l" notation.

lib/Driver/ToolChain.cpp

 std::string ToolChain::getCompilerRT(const ArgList &Args, StringRef Component,
                                      bool Shared) const {
   const llvm::Triple &TT = getTriple();
   const char *Env = TT.isAndroid() ? "-android" : "";
   bool IsITANMSVCWindows =
       TT.isWindowsMSVCEnvironment() || TT.isWindowsItaniumEnvironment();
 
   StringRef Arch = getArchNameForCompilerRTLib(*this, Args);
-  const char *Prefix = IsITANMSVCWindows ? "" : "lib";
-  const char *Suffix = Shared ? (Triple.isOSWindows() ? ".dll" : ".so")
+  const char *Prefix = "-l";
+  const char *Suffix = "";
+  SmallString<128> Path;
+
+  if (getDriver().SysRoot.empty()) {
+    Prefix = IsITANMSVCWindows ? "" : "lib";
+    Suffix = Shared ? (Triple.isOSWindows() ? ".dll" : ".so")
                               : (IsITANMSVCWindows ? ".lib" : ".a");
+    Path = getCompilerRTPath();
+  }
 
-  SmallString<128> Path(getCompilerRTPath());
   llvm::sys::path::append(Path, Prefix + Twine("clang_rt.") + Component + "-" +
                                     Arch + Env + Suffix);
   return Path.str();
 }
 
 const char *ToolChain::getCompilerRTArgString(const llvm::opt::ArgList &Args,
                                               StringRef Component,
                                               bool Shared) const {