glsl: Slightly change the semantic of _LinkedShaders

Previously _LinkedShaders was a compact array of the linked shaders
for each shader stage.  Now it is arranged such that each slot,
indexed by the MESA_SHADER_* defines, refers to a specific shader
stage.  As a result, some slots will be NULL.  This makes things a
little more complex in the linker, but it simplifies things in other
places.

As a side effect _NumLinkedShaders is removed.

NOTE: This may be a candidate for the 7.9 branch.  If there are other
patches that get backported to 7.9 that use _LinkedShader, this patch
should be cherry picked also.

src/glsl/glcpp/glcpp.c

@@ -29,9 +29,17 @@
 #include <errno.h>
 #include "glcpp.h"
 #include "main/mtypes.h"
+#include "main/shaderobj.h"
 
 extern int yydebug;
 
+void
+_mesa_reference_shader(struct gl_context *ctx, struct gl_shader **ptr,
+                       struct gl_shader *sh)
+{
+   *ptr = sh;
+}
+
 /* Read from fd until EOF and return a string of everything read.
  */
 static char *