Add VarDecl::isThisDeclarationADefinition(), which properly encapsulates the...

Add VarDecl::isThisDeclarationADefinition(), which properly encapsulates the logic for when a variable declaration is a (possibly tentativ) definition. Add a few functions building on this, and shift C tentative definition handling over to this new functionality. This shift also kills the Sema::TentativeDefinitions map and instead simply stores all declarations in the renamed list. The correct handling for multiple tentative definitions is instead shifted to the final walk of the list.

llvm-svn: 94968