[clangd] Cleanup: stop passing around list of supported URI schemes.

Summary:
Instead of passing around a list of supported URI schemes in clangd, we
expose an interface to convert a path to URI using any compatible scheme
that has been registered. It favors customized schemes and falls
back to "file" when no other scheme works.

Changes in this patch are:
- URI::create(AbsPath, URISchemes) -> URI::create(AbsPath). The new API finds a
compatible scheme from the registry.
- Remove URISchemes option everywhere (ClangdServer, SymbolCollecter, FileIndex etc).
- Unit tests will use "unittest" by default.
- Move "test" scheme from ClangdLSPServer to ClangdMain.cpp, and only
register the test scheme when lit-test or enable-lit-scheme is set.
(The new flag is added to make lit protocol.test work; I wonder if there
is alternative here.)

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D54800

llvm-svn: 347467