[Ada] Bit_Order cannot be defined for record extensions

[Pierre-Marie de Rodat]

This patch allows the compiler to report an error on Bit_Order when
defined for a record extension.

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

2018-07-16  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* sem_ch13.adb (Analyze_Attribute_Definition_Clause): Report an error
	on Bit_Order when defined for a record extension.

gcc/testsuite/

	* gnat.dg/bit_order1.adb: New testcase.
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