LibCpp: Import definitions from headers while processing

When the preprocessor encounters an #include statement it now adds
the preprocessor definitions that exist in the included header to its
own set of definitions.

We previously only aggregated the definitions from headers after
processing the source, which was less correct. (For example, there
could be an #ifdef that depends on a definition from another header).

Userland/Libraries/LibCpp/Parser.cpp

@@ -15,7 +15,7 @@
 
 namespace Cpp {
 
-Parser::Parser(Vector<Token> const& tokens, const String& filename, Preprocessor::Definitions&& definitions)
+Parser::Parser(Vector<Token> const& tokens, const String& filename, Preprocessor::Definitions const& definitions)
     : m_preprocessor_definitions(move(definitions))
     , m_filename(filename)
 {