libcxx: Rename vasprintf function to __libcpp_vasprintf.

The language standard does not define a function with this name,
so it is part of the user's namespace. This change fixes a duplicate
symbol error that occurs when a user attempts to define a function
with this name.

Differential Revision: https://reviews.llvm.org/D42405

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@323237 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed