[Ada] Reset Reachable field when mutating label into loop entity

[Ada] Reset Reachable field when mutating label into loop entity

An entity flag Reachable now only applies to E_Label entities and needs
to be explicitly reset when mutating labels into loop entities. Only
needed to prevent cascaded errors when compiling a malicious ACATS test
with -gnatq (try semantics, even if parse errors).

Cleanup related to detection of uninitialized scalars with GOTO
statements.

gcc/ada/

	* sem_ch5.adb (Analyze_Statements): Only clear Reachable flag on
	proper label entities.