Fix use-after-free bug in Tooling.

Summary:
`buildASTFromCodeWithArgs()` was creating a memory buffer referencing a
stack-allocated string.  This diff changes the implementation to copy the code
string into the memory buffer so that said buffer owns the memory.

Patch by Yitzhak Mandelbaum.

Reviewers: alexfh

Reviewed By: alexfh

Subscribers: cfe-commits, EricWF

Differential Revision: https://reviews.llvm.org/D55765

llvm-svn: 350638