ada: Fix source location for crashes in expanded Loop_Entry attributes

ada: Fix source location for crashes in expanded Loop_Entry attributes

Historically, Loop_Entry attributes were expanded while expanding their
corresponding loops, so it was easier to use location of these loops for
expanded code. Now, these attributes are expanded where they appear, so
we can easily use the location of the attribute reference for expanded
code.

This matters when there is a crash in the expanded code, e.g. because of
a stack overflow in the declaration of an constant object that captures
the Loop_Entry prefix. Now backtrace will point to the source location
of the attribute, which is more helpful than the location of the loop.

gcc/ada/

	* exp_attr.adb (Expand_Loop_Entry_Attribute): Use location of the
	attribute reference, not of the loop statement.