Simplify handling of direct initializers by letting Sema::AddInitializerToDecl handle...

Simplify handling of direct initializers by letting Sema::AddInitializerToDecl handle conversions, instead of using Sema::ActOnCXXTypeConstructExpr.
Additional benefit is that diagnostics are the same for both direct-initialization and copy-initialization.

In the case of "int x( expression );":
-The Init expression of VarDecl 'x' will be the expression inside the parentheses.
-VarDecl::hasCXXDirectInitializer for VarDecl 'x' will return true to let clients distinguish from "int x = expression ;".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57219 91177308-0d34-0410-b5e6-96231b3b80d8