glsl: Prohibit illegal mixing of redeclarations inside/outside gl_PerVertex.

From section 7.1 (Built-In Language Variables) of the GLSL 4.10
spec:

    Also, if a built-in interface block is redeclared, no member of
    the built-in declaration can be redeclared outside the block
    redeclaration.

We have been regarding this text as a clarification to the behaviour
established for gl_PerVertex by GLSL 1.50, so we apply it regardless
of GLSL version.

This patch enforces the rule by adding an enum to ir_variable to track
how the variable was declared: implicitly, normally, or in an
interface block.

Fixes piglit tests:
- gs-redeclares-pervertex-out-after-global-redeclaration.geom
- vs-redeclares-pervertex-out-after-global-redeclaration.vert
- gs-redeclares-pervertex-out-after-other-global-redeclaration.geom
- vs-redeclares-pervertex-out-after-other-global-redeclaration.vert
- gs-redeclares-pervertex-out-before-global-redeclaration
- vs-redeclares-pervertex-out-before-global-redeclaration

Cc: "10.0" <mesa-stable@lists.freedesktop.org>

v2: Don't set "how_declared" redundantly in builtin_variables.cpp.
Properly clone "how_declared".

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/ir.cpp

@@ -1586,7 +1586,8 @@ ir_variable::ir_variable(const struct glsl_type *type, const char *name,
 			 ir_variable_mode mode)
    : max_array_access(0), max_ifc_array_access(NULL),
      read_only(false), centroid(false), invariant(false),
-        mode(mode), interpolation(INTERP_QUALIFIER_NONE), atomic()
+     how_declared(ir_var_declared_normally), mode(mode),
+     interpolation(INTERP_QUALIFIER_NONE), atomic()
 {
    this->ir_type = ir_type_variable;
    this->type = type;