Multiple small fixes for better C conformance.

Allow atomic types to be initialized with single-element expression lists:
  int x = { 5 };
Issue an error if a storage class is provided with a function parameter.
Issue an error if two members of a struct have the same name.
Issue an error on trying to assign to a struct with a const member, even if
  the struct itself isn't const.
Issue an error if a function is redefined.
Issue an error if a function overload is declared that differs only in return
  type from a previously-declared function.
Issue an error if "inline" or "task" qualifiers are used outside of function
  declarations.
Allow trailing ',' at the end of enumerator lists.
Multiple tests for all of the above.

tests_errors/array-pointer-assign.ispc

@@ -1,4 +1,4 @@
-// ffofoof
+// Illegal to assign to array type "float[5]"
 
 void foo(float *x) {
     float a[5] = { 1,2,3,4,5};