Changed -serialize-ast to not create a temporary directory, but instead

Changed -serialize-ast to not create a temporary directory, but instead
create a .ast file in the current working directory.  This mirrors the behavior
of the -c option for gcc.  Later we should add the ability to write the
serialized file anywhere.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@45004 91177308-0d34-0410-b5e6-96231b3b80d8