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_errors/addr-of-1.ispc

@@ -0,0 +1,5 @@
+// Illegal to take address of non-lvalue or function
+
+void foo() {
+    int *ptr = &(1+1);
+}