PP: Don't report certain error about '#' when #if'd out

Don't report the following error when scanning inactive code (e.g. code inside #if 0):
"error: '#' : (#) can be preceded in its line only by spaces or horizontal tab"
Adds a variable to PpContext to say whether we're currently skipping over an inactive #if/#ifdef/#elif/#else, and don't report the error inside scanToken if true.

fixes 3704

Test/preprocess.inactive_stringify.vert

@@ -0,0 +1,28 @@
+#version 460
+
+// This tests that the preprocessor error
+// "error: '#' : (#) can be preceded in its line only by spaces or horizontal tab"
+// isn't enforced when inactive (e.g. inside #if 0)
+
+#if 0
+#define STRINGIFY(X) #X
+#endif
+
+#define C 0
+
+#if 1
+#ifdef A
+#elif defined B
+#elif C
+// OK, since preprocessor evaluates to inactive
+#define STRINGIFY(X) #X
+#endif
+#endif
+
+// OK in comments
+// #define STRINGIFY(X) #X
+
+void main()
+{
+    gl_Position = vec4(1.0);
+}