glcpp: Add test case for recently fixed loop-control underflow bug.

To trigger the bug, it suffices to have a line-continuation followed by
a newline and then a non-line-continuation backslash.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/glcpp/tests/117-line-continuation-and-non-continuation-backslash.c

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+/* This test case is the minimal case to replicate the bug reported here:
+ *
+ * https://bugs.freedesktop.org/show_bug.cgi?id=65112
+ *
+ * To trigger the bug, there must be a line-continuation sequence
+ * (backslash newline), then an additional newline character, and
+ * finally another backslash that is not part of a line-continuation
+ * sequence.
+ */
+\
+
+/* \ */

src/glsl/glcpp/tests/117-line-continuation-and-non-continuation-backslash.c.expected

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+