glcpp: error on multiple #else/#elif directives

The preprocessor currently accepts multiple else/elif-groups
per if-section. The GLSL-preprocessor is defined by the C++
specification, which defines the following parse-rule:

if-section:
	if-group elif-groups(opt) else-group(opt) endif-line

This clearly only allows a single else-group, that has to come
after any elif-groups.

So let's modify the code to follow the specification. Add test
to prevent regressions.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Carl Worth <cworth@cworth.org>

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

src/glsl/glcpp/glcpp.h

@@ -153,6 +153,7 @@ typedef enum skip_type {
 
 typedef struct skip_node {
 	skip_type_t type;
+	bool has_else;
 	YYLTYPE loc; /* location of the initial #if/#elif/... */
 	struct skip_node *next;
 } skip_node_t;