[Ada] Performance of CW_Membership

[Pierre-Marie de Rodat]

CM_Membership is used implicitly by some tagged related constructs, and
having it never inlined may cause performance issues, so move it to the
spec by taking advantage of the recently added declare expressions.

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

gcc/ada/

	* libgnat/a-tags.ads, libgnat/a-tags.adb (CW_Membership): Move
	to spec to allow inlining.

gcc/testsuite/

	* gnat.dg/debug15.adb: Remove fragile testcase.
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