Modify some deleted function methods to better reflect reality:

Modify some deleted function methods to better reflect reality:

 - New isDefined() function checks for deletedness
 - isThisDeclarationADefinition checks for deletedness
 - New doesThisDeclarationHaveABody() does what
   isThisDeclarationADefinition() used to do
 - The IsDeleted bit is not propagated across redeclarations
 - isDeleted() now checks the canoncial declaration
 - New isDeletedAsWritten() does what it says on the tin.
 - isUserProvided() now correct (thanks Richard!)

This fixes the bug that we weren't catching

void foo() = delete;
void foo() {}

as being a redefinition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@131013 91177308-0d34-0410-b5e6-96231b3b80d8