Extend the syntax of #line and __FILE__ to support filename strings.

According to the GLSL spec, the second parameter to #line should be
an integer source string number and __FILE__ will be substituted
with the integer source string number currently processed. This
patch extends the syntax of #line and __FILE__. Now #line accepts
as the second parameter a filename string quoted by double quotation
marks. And if such a #line is set, __FILE__ will be substituted with
the currently set filename string. The implementation is done via
introducing a new extension GL_GOOGLE_cpp_style_line_directive using
the extension framework.

The purpose is to support cpp-style #line directives, which is
required by #include.

Test/preprocessor.cpp_style_line_directive.vert

@@ -0,0 +1,36 @@
+#extension GL_GOOGLE_cpp_style_line_directive : enable
+
+#error at "0:3"
+
+#line 150 "a.h"
+#error at "a.h:150"
+
+#line 24
+#error at "a.h:24"
+
+#line 42
+#error at "a.h:42"
+
+#line 30 "b.cc"
+#error at "b.cc:30"
+
+#line 10 3
+#error at "3:10"
+
+#line 48
+#error at "3:48"
+
+#line 4
+#error at "3:4"
+
+#line 55 100
+#error at "100:55"
+
+#line 1000 "c"
+#error at "c:1000"
+
+#line 42 1
+#error at "1:42"
+
+#line 42 "this-is-a-quite-long-name-maybe-i-should-shorten-it"
+#error at "this-is-a-quite-long-name-maybe-i-should-shorten-it:42"