Modified the internals of Diagnostic and DiagnosticClient to use

SourceManager*'s instead of SourceManager&'s. This allows the client specify a
NULL SourceManager when using a default constructed SourceLocation. Thus the
SourceManager can be NULL when the SourceLocation's isValid() == false.

The interface to most clients of Diagnostic remains the same.
Diagnostic::Report() is overload to either accept a SourceLocation and a
SourceManager&, or neither. Thus clients that do not have a SourceManager
cannot specify a SourceLocation.

Modified TextDiagnostics* to use this new interface.
Modified the driver to not passed in SourceManager when warning about "-I-".

llvm-svn: 44887