Preprocessor: Allow '\' as a token, which gets actual use in #error, but should have no other use.

This is just for '\' that's not before a new line.
Note the specification says it has no use other than as line continuation,
but #error is a grey area. (There are no escape sequences.)

glslang/MachineIndependent/preprocessor/PpAtom.cpp

@@ -175,7 +175,7 @@ void TPpContext::AddAtomFixed(const char* s, int atom)
 void TPpContext::InitAtomTable()
 {
     // Add single character tokens to the atom table:
-    const char* s = "~!%^&*()-+=|,.<>/?;:[]{}#";
+    const char* s = "~!%^&*()-+=|,.<>/?;:[]{}#\\";
     char t[2];
 
     t[1] = '\0';