Allow to store precompiled preambles in memory.

Summary:
These preambles are built by ASTUnit and clangd. Previously, preambles
were always stored on disk.

In-memory preambles are routed back to the compiler as virtual files in
a custom VFS.

Interface of ASTUnit does not allow to use in-memory preambles, as
ASTUnit::CodeComplete receives FileManager as a parameter, so we can't
change VFS used by the compiler inside the CodeComplete method.

A follow-up commit will update clangd in clang-tools-extra to use
in-memory preambles.

Reviewers: klimek, sammccall, bkramer

Reviewed By: klimek

Subscribers: ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D39842

llvm-svn: 318411