[Ada] Fix ineffective -gnatyN for separate compilation units

[Pierre-Marie de Rodat]

This fixes a recent regression introduced in the machinery giving style
warnings: the -gnatyN switch no longer cancels an earlier -gnaty switch
for separate compilation units.

Running this command:

  gcc -c slib.adb -gnatyaAbcefhiIklmM25OnprStux -gnatyN

On the following sources:

package Slib is
   procedure I_Am_Short;
end Slib;

with Ada.Text_IO; use Ada.Text_IO;

package body Slib is
   --  have a long line here as well.
   procedure                I_Am_Short is               separate;
end Slib;

separate (Slib)

procedure I_Am_Short is
begin
    Put_Line (".......................................................");
end I_Am_Short;

Should execute silently.

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

2019-09-17  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem.adb (Do_Analyze): Save Style_Check_Max_Line_Length on
	entry and restore it on exit instead of recomputing it.
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