Refactor Objective-C @catch parameter checking by detangling it from

Refactor Objective-C @catch parameter checking by detangling it from
function-parameter checking and splitting it into the normal
ActOn*/Build* pair in Sema. We now use VarDecl to represent the @catch
parameter rather than the ill-fitting ParmVarDecl.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@102347 91177308-0d34-0410-b5e6-96231b3b80d8