Fix a bug that swift clang importer is not setup correctly when prefix
map is used. There are two separate issues:
* CC1 args used to setup clang import when building swift (interface and
sources) are not correctly remapped.
* When loading SDKInfo from SDK path, the SDKSettings.json is not
loading from VFS, thus the file cannot be loaded from remapped path.
rdar://134458611
When dependency scanner construct the overlay file path for the
swift-frontend invocation, make sure the path is remapped if prefix map
is used.
rdar://131940130
Stop relying on file system to provide source buffer for diagnostics
when replying. This avoids initializing CASFS which is quite expensive.
Now cached diagnostics contains CASID for the file buffer so it can
initialize its own source manager without relying on the underlying file
system.
rdar://128423393
