[Ada] Initialize Compiler_State to avoid Constraint_Error

[Ada] Initialize Compiler_State to avoid Constraint_Error

When building gnat1 with `-gnatVa` as we do locally, rules like: `gcc -c
-gnatyM79 <any adb file with lines longer than 79>` will throw a
constraint error as `lib.compiler_state` is initialized by par.adb, ie
after scanning. Therefore any error_msg thrown during scanning will
perform this uninitialized read (which raises a Constraint_Error when
the compiler was compiled with `-gnatVa`).

Initialize this flag to `Parsing`.

gcc/ada/

	* lib.ads: initialize `Compiler_State` to `Parsing`.