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/baseResults/preprocess.inactive_stringify.vert.out

@@ -0,0 +1,29 @@
+#version 460
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+void main()
+{
+    gl_Position = vec4(1.0);
+}
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