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

test/SILGen/Inputs/usr/include/Ansible.h

@@ -4,8 +4,14 @@
 - (Ansible*)initWithBellsOn:(id)theBells;
 @end
 
-Ansible *NSAnse(Ansible *x) {
+static Ansible *NSAnse(Ansible *x) {
+  return x;
+}
+
+Ansible *NSAnseExternal(Ansible *x) {
   return x;
 }
 
 void hasNoPrototype();
+
+static void staticForwardDeclaration(void);