Now that multiple prefixes are much cheaper to search for GCC

installations, support them when installed directly under the system
root ('/lib/gcc/...' essentially).

With this, Clang can correctly detect and use a cross-compiling GCC
installation within a system root and use it.

Again, test cases will be coming in later commits, as I'm going to write
a few test cases that exercise nearly all of this logic.

llvm-svn: 141121

clang/lib/Driver/ToolChains.cpp

@@ -1639,6 +1639,7 @@ public:
     // Compute the set of prefixes for our search.
     SmallVector<std::string, 8> Prefixes(D.PrefixDirs.begin(),
                                          D.PrefixDirs.end());
+    Prefixes.push_back(D.SysRoot);
     Prefixes.push_back(D.SysRoot + "/usr");
 
     // Loop over the various components which exist and select the best GCC