Change if_then_else to be inline

Users will expect it to be comparable in performance to the ternary
conditional operator, so we should do it that way.

drake/common/double_overloads.cc

@@ -1,8 +1,4 @@
 #include "drake/common/double_overloads.h"
 
-namespace drake {
-double if_then_else(const bool f_cond, const double v_then,
-                    const double v_else) {
-  return f_cond ? v_then : v_else;
-}
-}  // namespace drake
+// For now, this is an empty .cc file that only serves to confirm that
+// double_overloads.h is a stand-alone header.

drake/common/double_overloads.h

@@ -14,5 +14,8 @@ namespace drake {
 /// v_else are evaluated regardless of the evaluation of @p f_cond. In contrast,
 /// only one of @p v_then or @p v_else is evaluated in C++'s conditional
 /// expression <tt>f_cond ? v_then : v_else</tt>.
-double if_then_else(bool f_cond, double v_then, double v_else);
+inline double if_then_else(bool f_cond, double v_then, double v_else) {
+  return f_cond ? v_then : v_else;
+}
+
 }  // namespace drake