External interface change: PP: Full <> and "" semantics for the Includer.

Any previous use would only be for "", which would probably mean changing

    include(...)  -> includeLocal(...)

See comments about includeLocal() being an additional search over
includeSystem(), not a superset search.

This also removed ForbidIncluder, as
 - the message in ForbidIncluder was redundant: error results were
   already returned to the caller, which then gives the error it
   wants to
 - there is a trivial default implementation that a subclass can
   override any subset of (I still like abstract base classes though)
 - trying to get less implementation out of the interface file anyway

gtests/TestFixture.h

@@ -521,7 +521,7 @@ public:
         glslang::TShader shader(EShLangVertex);
         shader.setStringsWithLengths(&shaderStrings, &shaderLengths, 1);
         std::string ppShader;
-        glslang::TShader::ForbidInclude includer;
+        glslang::TShader::Includer includer;
         const bool success = shader.preprocess(
             &glslang::DefaultTBuiltInResource, defaultVersion, defaultProfile,
             forceVersionProfile, isForwardCompatible, (EShMessages)(EShMsgOnlyPreprocessor | EShMsgCascadingErrors),