Implement support for C++ direct initializers in declarations, e.g. "int x(1);".

This is how this kind of initializers appear in the AST:
-The Init expression of the VarDecl is a functional type construction (of the VarDecl's type).
-The new VarDecl::hasCXXDirectInitializer() returns true.

e.g, for "int x(1);":
-VarDecl 'x' has Init with expression "int(1)" (CXXFunctionalCastExpr).
-hasCXXDirectInitializer() of VarDecl 'x' returns true.

A major benefit is that clients that don't particularly care about which exactly form was the initializer can handle both cases without special case code.
Note that codegening works now for "int x(1);" without any changes to CodeGen.

llvm-svn: 57178