[ast] CreateParameterDeclaration should use an appropriate DeclContext.

[ast] CreateParameterDeclaration should use an appropriate DeclContext.

Previously CreateParameterDeclaration was always using the translation
unit DeclContext.  We would later go and add parameters to the
FunctionDecl, but internally clang makes a copy when you do this, and
we'd end up with ParmVarDecl's at the global scope as well as in the
function scope.

This fixes the issue.  It's hard to say whether this will introduce
a behavioral change in name lookup, but I know there have been several
hacks introduced in previous years to deal with collisions between
various types of variables, so there's a chance that this patch could
obviate one of those hacks.

Differential Revision: https://reviews.llvm.org/D55571

llvm-svn: 348941