Sema: Create a no-op implicit cast for lvalue function conversions.

This fixes an assertion failure in the case where an implicit conversion for a
function call involves an lvalue function conversion, and makes the AST for
initializations involving implicit lvalue function conversions more accurate.

Differential Revision: https://reviews.llvm.org/D66437

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@375313 91177308-0d34-0410-b5e6-96231b3b80d8