[Ada] Deconstruct 'F' as a prefix for an ALI data

[Ada] Deconstruct 'F' as a prefix for an ALI data

In GNATprove we used to store a variant of cross-reference information in
the ALI file in lines that started with an 'F' letter. This is no longer
the case, so the letter can be returned to the pool of unused prefixes.

2018-07-31  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* ali.adb (Known_ALI_Lines): Remove 'F' as a prefix for lines
	related to the FORMAL analysis done by GNATprove.

From-SVN: r263092