[Serialization] Place command line defines in the correct file
Fix several problems related to serialization causing command line
defines to be reported as being built-in defines:
* When serializing the <built-in> and <command line> files don't
convert them into absolute paths.
* When deserializing SM_SLOC_BUFFER_ENTRY we need to call
setHasLineDirectives in the same way as we do for
SM_SLOC_FILE_ENTRY.
* When created suggested predefines based on the current command line
options we need to add line markers in the same way that
InitializePreprocessor does.
* Adjust a place in clangd where it was implicitly relying on command
line defines being treated as builtin.
Differential Revision: https://reviews.llvm.org/D144651