Following up the earlier refactoring/cleanup work by fixing up how we manage the...

Following up the earlier refactoring/cleanup work by fixing up how we manage the virtual files the ASTReader has to handle.  Specifically, this occurs when the reader is reading AST files that were created in memory and not written to disk.  For example, when a user creates a chained PCH using command line flags.  These virtual files are stored in MemoryBuffers in ChainIncludeSource.cpp, and then read back in by the ASTReader.  This patch moves the management of these buffers into the ModuleManager, so that it becomes the authority on where these buffers are located.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@136697 91177308-0d34-0410-b5e6-96231b3b80d8