Fixed a problem where the ASTImporter mishandled in-class initializers.

Previously, the ASTImporter, when copying a FieldDecl, would make the
new FieldDecl use the exact same in-class initializer as the original
FieldDecl, which is a problem since the initializer is in the wrong AST.
The initializer must be imported, just like all the other parts of the
field.

Doug Gregor reviewed this fix.

<rdar://problem/24943405>

llvm-svn: 262572