[Ada] Spurious dimensionality errors on multitidimensional aggregates.

[Arnaud Charlet <charlet at adacore dot com>]

This patch removes spurious dimensionality errors on aggregates for multi-
dimensional arrays of scalar types with dimension specifications.

Compiling dimbug.ads below must yield:

   dimbug.ads:8:33: dimensions mismatch in array aggregate
   dimbug.ads:8:33: expected dimension [L], found [M]

---
with system.Dim.Mks; use System.Dim.Mks;
package dimbug is

 test_array_2 : array (1 .. 3, 1 .. 3) of Length :=
      (others => (others => 3.0 * m));    --  OK

 Bad_Array : array (1 .. 3, 1 .. 3) of Length :=
      (others => (others => 3.0 * kg));   --  ERROR
end;

Tested on x86_64-pc-linux-gnu, committed on trunk

2016-04-27  Ed Schonberg  <schonberg@adacore.com>

	* sem_dim.ads, sem_dim.adb (Check_Expression_Dimensions): New
	procedure to compute the dimension vector of a scalar expression
	and compare it with the dimensions if its expected subtype. Used
	for the ultimate components of a multidimensional aggregate,
	whose components typically are themselves aggregates that are
	expanded separately. Previous to this patch, dimensionality
	checking on such aggregates generated spurious errors.
	* sem_aggr.adb (Resolve_Array_Aggregate): Use
	Check_Expression_Dimensions when needed.

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