class Preprocessor: Now owns the "predefines" char*; it deletes [] it in its dstor.

clang.cpp: InitializePreprocessor now makes a copy of the contents of PredefinesBuffer and
  passes it to the preprocessor object.
  
clang.cpp: DriverPreprocessorFactory now calls "InitializePreprocessor" instead of this being done in main().

html::HighlightMacros() now takes a PreprocessorFactory, allowing it to conjure up a new
Preprocessor to highlight macros.

class HTMLDiagnostics now takes a PreprocessorFactory* that it can use for html::HighlightMacros().
Updated clients of HTMLDiagnostics to use this new interface.

llvm-svn: 49875