#1559 Enhancements in clangd-client handling of external makefile projects

[Christo]

Please find patch attached with following enhancements in clangd-client handling of external makefile projects.
1. Avoid deleting compilation db on reparse. For external makefiles, compilation db also probably will be copied by the user, so keep it.
2. Support relative path of files when checking if file is already present in db. Some tools generate compilation db with relative paths.
3. Do not call AddFileToCompileDBJson if file is present in compilation db.
4. Remove external makefile specific code from AddFileToCompileDBJson as it is covered by #3

[pecan]

Applied rev 13750
Thanks