Check if LLVM_PREFIX is defined before using it.

[chaoren]

Summary

LLVM_PREFIX could be undefined if CMAKE_INSTALL_PREFIX were set to empty.

[kparzysz]

This seemed fine to me, but at the second thought, the lines 2610-2613 in the original should be removed.

[kparzysz]

This seemed fine to me, but at the second thought, the lines 2610-2613 in the original should be removed.

[chandlerc]

In D17784#367601, @kparzysz wrote:

This seemed fine to me, but at the second thought, the lines 2610-2613 in the original should be removed.

I agree. I think using LLVM_PREFIX here in any capacity is incorrect.

[chaoren]

Remove usage of LLVM_PREFIX altogether.

lib/Driver/ToolChains.cpp

std::string HexagonToolChain::getHexagonTargetDir(
   // Locate the rest of the toolchain ...
   for (auto &I : PrefixDirs)
     if (D.getVFS().exists(I))
       return I;
 
   if (getVFS().exists(InstallRelDir = InstalledDir + "/../target"))
     return InstallRelDir;
 
+#ifdef LLVM_PREFIX
   std::string PrefixRelDir = std::string(LLVM_PREFIX) + "/target";
   if (getVFS().exists(PrefixRelDir))
     return PrefixRelDir;
+#endif
 
   return InstallRelDir;
 }
 
 Optional<unsigned> HexagonToolChain::getSmallDataThreshold(
       const ArgList &Args) {
   StringRef Gn = "";
   if (Arg *A = Args.getLastArg(options::OPT_G, options::OPT_G_EQ,