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

glslang/MachineIndependent/preprocessor/PpContext.cpp

@@ -88,7 +88,8 @@ TPpContext::TPpContext(TParseContextBase& pc, const std::string& rootFileName, T
     preamble(nullptr), strings(nullptr), previous_token('\n'), parseContext(pc), includer(inclr), inComment(false),
     rootFileName(rootFileName),
     currentSourceFile(rootFileName),
-    disableEscapeSequences(false)
+    disableEscapeSequences(false),
+    inElseSkip(false)
 {
     ifdepth = 0;
     for (elsetracker = 0; elsetracker < maxIfNesting; elsetracker++)