Fix linkage of 'static inline' Clang-imported definitions

If an external SIL function has a Clang-generated body, I think this
means we have a static function, and we want to use Shared linkage,
not Public.

Add a new flag to SILFunction for this and plumb it through to
appease assertions from SILVerifier.

Swift SVN r31763

lib/SIL/SILFunction.cpp

@@ -89,6 +89,7 @@ SILFunction::SILFunction(SILModule &Module, SILLinkage Linkage,
     InlineStrategy(inlineStrategy),
     Linkage(unsigned(Linkage)),
     KeepAsPublic(false),
+    ForeignBody(false),
     EK(E) {
   if (InsertBefore)
     Module.functions.insert(SILModule::iterator(InsertBefore), this);