Revamp handling of function types, conversion to function ptr types.

Implicit conversion to function types is now a more standard part of
the type conversion infrastructure, rather than special cases of things
like FunctionSymbolExpr immediately returning a pointer type, etc.

Improved AddressOfExpr::TypeCheck() to actually issue errors in cases
where it's illegal to take the address of an expression.

Added AddressOfExpr::GetConstant() implementation that handles taking
the address of functions.

Issue #223.

tests/funcptr-null-1.ispc

@@ -15,7 +15,7 @@ export void f_f(uniform float RET[], uniform float aFOO[]) {
     float a = aFOO[programIndex]; 
     float b = aFOO[0]-1;
     uniform FuncType func = foo;
-    RET[programIndex] = (func ? func : bar)(a, b);
+    RET[programIndex] = (func ? func : &bar)(a, b);
 }
 
 export void result(uniform float RET[]) {

tests/funcptr-null-3.ispc

@@ -14,7 +14,7 @@ static float bar(float a, float b) {
 export void f_f(uniform float RET[], uniform float aFOO[]) {
     float a = aFOO[programIndex]; 
     float b = aFOO[0]-1;
-    FuncType func = foo;
+    FuncType func = &foo;
     if (a == 2)
         func = NULL;
     if (func != NULL)

tests/funcptr-null-6.ispc

@@ -16,7 +16,7 @@ export void f_f(uniform float RET[], uniform float aFOO[]) {
     float b = aFOO[0]-1;
     FuncType func = NULL;
     if (a == 2)
-        func = foo;
+        func = &foo;
     if (!func)
         RET[programIndex] = -1;
     else

tests/funcptr-uniform-2.ispc

@@ -14,7 +14,7 @@ static float bar(float a, float b) {
 export void f_f(uniform float RET[], uniform float aFOO[]) {
     float a = aFOO[programIndex]; 
     float b = aFOO[0]-1;
-    uniform FuncType func = bar;
+    uniform FuncType func = &bar;
     if (aFOO[0] == 1)
         func = foo;
     RET[programIndex] = func(a, b);