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.)

Test/baseResults/cppSimple.vert.out

@@ -82,8 +82,17 @@ ERROR: 12:9014: '=' :  cannot convert from 'temp float' to 'global int'
 ERROR: 12:9015: 'macro expansion' : expected '(' following FOOOM
 ERROR: 12:9016: 'preprocessor evaluation' : can't evaluate expression 
 ERROR: 12:9016: 'preprocessor evaluation' : bad expression 
+ERROR: 12:9500: 'preprocessor evaluation' : bad expression 
+ERROR: 12:9500: '#if' : unexpected tokens following directive 
+ERROR: 12:9502: 'preprocessor evaluation' : bad expression 
+ERROR: 12:9502: '#if' : unexpected tokens following directive 
+ERROR: 12:9504: 'preprocessor evaluation' : bad expression 
+ERROR: 12:9504: '#if' : unexpected tokens following directive 
+ERROR: 12:9506: '#error' : \ 377  
+ERROR: 12:9507: '#error' : \ 376  
+ERROR: 12:9508: '#error' : \ 377  
 ERROR: 12:10002: '' : missing #endif 
-ERROR: 79 compilation errors.  No code generated.
+ERROR: 88 compilation errors.  No code generated.
 
 
 Shader version: 400

Test/cppSimple.vert

@@ -326,6 +326,17 @@ int aoeua = FOOOM;
 #if FOOOM
 #endif
 
+#line 9500
+#if\376
+#endif
+#if \376
+#endif
+#if \377
+#endif
+#error\377
+#error \ 376
+#error \377
+
 #line 10000
 #if 1
 #else