Addition of TranslationUnitDecl to the AST:

Addition of TranslationUnitDecl to the AST:

-Added TranslationUnitDecl class to serve as top declaration context
-ASTContext gets a TUDecl member and a getTranslationUnitDecl() function
-All ScopedDecls get the TUDecl as DeclContext when declared at global scope

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@49855 91177308-0d34-0410-b5e6-96231b3b80d8