Basic: Add native support for stdin to SourceManager and FileManager

Add support for stdin to SourceManager and FileManager. Adds
FileManager::getSTDIN, which adds a FileEntryRef for `<stdin>` and reads
the MemoryBuffer, which is stored as `FileEntry::Content`.

Eventually the other buffers in `ContentCache` will sink to here as well
-- we probably usually want to load/save a MemoryBuffer eagerly -- but
it's happening early for stdin to get rid of
CompilerInstance::InitializeSourceManager's final call to
`SourceManager::overrideFileContents`.

clang/test/CXX/modules-ts/dcl.dcl/dcl.module/dcl.module.export/p1.cpp
relies on building a module from stdin; supporting that requires setting
ContentCache::BufferOverridden.

Differential Revision: https://reviews.llvm.org/D93148