[Ada] Fix access to uninitialized global variable when emitting error

[Pierre-Marie de Rodat]

When calling Error_Msg_FE with a message string that contains "<<"
insertion characters, the error reporting machinery was accessing
uninitialized Error_Msg_Warn variable to decide whether to emit error or
a warning.

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

gcc/ada/

	* sem_util.adb (Bad_Predicated_Subtype_Use): Emit an
	unconditional error, not a conditional warning.
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