ChainedIncludesSource: avoid copying n*(n+1)/2 file contents into memory

Just reference the data instead with shallow MemoryBuffer instances.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212450 91177308-0d34-0410-b5e6-96231b3b80d8
Move ChainedIncludesSource into the implementation

This doesn't need to be in the headers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212451 91177308-0d34-0410-b5e6-96231b3b80d8