Improve error checking for unsized arrays.

Added support for resolving dimensions of multi-dimensional unsized arrays
from their initializer exprerssions (previously, only the first dimension
would be resolved.)

Added checks to make sure that no unsized array dimensions remain after
doing this (except for the first dimensision of array parameters to
functions.)

tests_errors/initexpr-4.ispc

@@ -0,0 +1,4 @@
+// Illegal to declare an unsized array variable without providing an initializer expression to set its size
+
+void foo() { int a[][]; }
+