InputCommon/FunctionExpression: Collapse namespaces

Since we target C++17, we can collapse the namespaces into a single
declaration specifier.

Source/Core/InputCommon/ControlReference/FunctionExpression.cpp

@@ -8,9 +8,7 @@
 #include <chrono>
 #include <cmath>
 
-namespace ciface
-{
-namespace ExpressionParser
+namespace ciface::ExpressionParser
 {
 constexpr int LOOP_MAX_REPS = 10000;
 constexpr ControlState CONDITION_THRESHOLD = 0.5;
@@ -523,5 +521,4 @@ void FunctionExpression::SetValue(ControlState)
 {
 }
 
-}  // namespace ExpressionParser
-}  // namespace ciface
+}  // namespace ciface::ExpressionParser

Source/Core/InputCommon/ControlReference/FunctionExpression.h

@@ -12,9 +12,7 @@
 #include "InputCommon/ControlReference/ExpressionParser.h"
 #include "InputCommon/ControlReference/FunctionExpression.h"
 
-namespace ciface
-{
-namespace ExpressionParser
+namespace ciface::ExpressionParser
 {
 class FunctionExpression : public Expression
 {
@@ -51,5 +49,4 @@ private:
 
 std::unique_ptr<FunctionExpression> MakeFunctionExpression(std::string name);
 
-}  // namespace ExpressionParser
-}  // namespace ciface
+}  // namespace ciface::ExpressionParser