Bug fix: Emit code for C++ inline function called from another inline function.

Adds a test that fails without the fix.

The fix consists simply of removing a return statement; as a result, we
now always emit code for functions referenced from the function we are
currently emitting.

Previously, this was not done for externally visible functions. This
goes all the way back to when `GenClangDecl.cpp` was originally introduced here:

swiftlang@ee22004

I speculate that the reason for doing this was that in C and Objective-C, we
can assume that an externally visible function will be provided by some
`.o` file we're linking against (though I'm not sure if this is in fact
true for C99 inline functions). At any rate, this assumption is no longer true
for C++ inline functions.

Author: martinboehme

lib/IRGen/GenClangDecl.cpp

@@ -41,7 +41,6 @@ void IRGenModule::emitClangDecl(const clang::Decl *decl) {
   if (!valueDecl || valueDecl->isExternallyVisible()) {
     ClangCodeGen->HandleTopLevelDecl(
                           clang::DeclGroupRef(const_cast<clang::Decl*>(decl)));
-    return;
   }
 
   if (!GlobalClangDecls.insert(decl->getCanonicalDecl()).second)

test/Interop/Cxx/function/Inputs/EmitCalledFunction.h

@@ -0,0 +1,7 @@
+inline int bar() {
+  return 42;
+}
+
+inline int foo() {
+  return bar();
+}

test/Interop/Cxx/function/Inputs/module.modulemap

@@ -0,0 +1,4 @@
+module EmitCalledFunction {
+  header "EmitCalledFunction.h"
+  export *
+}

test/Interop/Cxx/function/emit-called-function-ir.swift

@@ -0,0 +1,14 @@
+// This is a reproducer for a bug where Swift was not emitting code for a
+// C++ inline function called from another inline function.
+//
+// In our example, inline function foo() calls inline function bar(). The bug
+// resulted in bar() not being emitted.
+
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -I %S/Inputs -enable-cxx-interop -emit-ir -o - | %FileCheck %s
+
+import EmitCalledFunction
+
+// CHECK-DAG: define linkonce_odr i32 @_Z3foov() #{{[0-9]+}} comdat {
+// CHECK-DAG: define linkonce_odr i32 @_Z3barv() #{{[0-9]+}} comdat {
+foo()