Fix multiple bugs related to DIBuilder::createFunction() call.

The DIType passed to this method should correspond to the
FunctionType of the function, not its return type.

The first parameter should be the DIScope for the compile unit,
not the DIFile.

We previously had the unmangled function name and the mangled
function name interchanged.

The argument corresponding to "first line number of the function" was
missing, which in turn led to subsequent arguments being off, and thus
providing bogus values vs. what was supposed to be passed.

Rename FunctionEmitContext::diFunction to diSubprogram, to better
reflect its type.

ctx.h

@@ -641,7 +641,7 @@ private:
 
     /** DISubprogram corresponding to this function (used for debugging
         info). */
-    llvm::DISubprogram diFunction;
+    llvm::DISubprogram diSubprogram;
 
     /** These correspond to the current set of nested scopes in the
         function. */