Add a new warning to notify users of mismatched SDK and deployment target

Summary:
This patch adds a new driver warning -Wincompatible-sdk which notifies the user when they are mismatching the version min options and the sysroot.

The patch works by checking the sysroot (if present) for an SDK name, then matching that against the target platform. In the case of a mismatch it logs a warning.

Reviewers: bob.wilson, rsmith

Subscribers: rsmith, edward-san, cfe-commits

Differential Revision: http://reviews.llvm.org/D18088

llvm-svn: 268127
diff --git a/clang/lib/Driver/ToolChains.h b/clang/lib/Driver/ToolChains.h
index e799217..9a08d8e 100644
--- a/clang/lib/Driver/ToolChains.h
+++ b/clang/lib/Driver/ToolChains.h
@@ -496,6 +496,8 @@
     return TargetVersion < VersionTuple(V0, V1, V2);
   }
 
+  StringRef getPlatformFamily() const;
+  static StringRef getSDKName(StringRef isysroot);
   StringRef getOSLibraryNameSuffix() const;
 
 public: