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

gtests/Pp.FromFile.cpp

@@ -69,6 +69,7 @@ INSTANTIATE_TEST_SUITE_P(
         "preprocessor.eof_missing.vert",
         "preprocess.arb_shading_language_include.vert",
         "preprocess.include_directive_missing_extension.vert",
+        "preprocess.inactive_stringify.vert"
     })),
     FileNameAsCustomTestSuffix
 );