[ExpressionParser] Reuse the FileManager from the compiler instance.

I was looking at the ClangExpressionParser and noticed that we have a
FileManager owned by the expression parser and later ask the compiler
instance to create a new FileManager, owned by the clang CI. Looking at
the code I don't see a good reason for having two instances. This patch
removes the one owned by LLDB.

Differential revision: https://reviews.llvm.org/D58222

llvm-svn: 354041