[Ada] Fixes for pretty command-line GNATprove output with -gnatdF

[Pierre-Marie de Rodat]

Various fixes are applied to the recent pretty output mode for GNATprove,
activated under debug switch -gnatdF:
- do not separate info messages from previous ones
- do not display souce code line for info messages
- display source code lines closer to the format adopted in GCC
- do not set the exit status to error when only check messages are issued

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

gcc/ada/

	* errout.adb (Write_Source_Code_Line): Adopt display closer to
	GCC format.
	(Output_Messages): Deal specially with info messages.
	* erroutc.adb (Prescan_Message): Fix bug leading to check
	messages being considered as error messages in pretty output
	mode.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: patch.diff
Type: text/x-diff
Size: 7202 bytes
Desc: not available
URL: <https://gcc.gnu.org/pipermail/gcc-patches/attachments/20201020/cf2c992e/attachment-0001.bin>