[Ada] Initialize Compiler_State to avoid Constraint_Error

[Pierre-Marie de Rodat via Gcc-patches]

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`.

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

gcc/ada/

        * lib.ads: initialize `Compiler_State` to `Parsing`.
diff --git a/gcc/ada/lib.ads b/gcc/ada/lib.ads
--- a/gcc/ada/lib.ads
+++ b/gcc/ada/lib.ads
@@ -39,7 +39,7 @@ package Lib is
    --  Type to hold list of indirect references to unit number table
 
    type Compiler_State_Type is (Parsing, Analyzing);
-   Compiler_State : Compiler_State_Type;
+   Compiler_State : Compiler_State_Type := Parsing;
    --  Indicates current state of compilation. This is used to implement the
    --  function In_Extended_Main_Source_Unit.