Fix PR20773 which I introduced with a silly edit mistake in r216531.
Trivial fix, and I've made the gentoo tests more representative. With
the changes, they would have caught this failure.

llvm-svn: 216572
diff --git a/clang/lib/Driver/ToolChains.cpp b/clang/lib/Driver/ToolChains.cpp
index 9ea21b4..2f47115 100644
--- a/clang/lib/Driver/ToolChains.cpp
+++ b/clang/lib/Driver/ToolChains.cpp
@@ -3353,7 +3353,7 @@
                                                 Twine IncludeSuffix,
                                                 const ArgList &DriverArgs,
                                                 ArgStringList &CC1Args) {
-  if (!llvm::sys::fs::exists(Base))
+  if (!llvm::sys::fs::exists(Base + Suffix))
     return false;
 
   addSystemInclude(DriverArgs, CC1Args, Base + Suffix);