[clang][FileManager] Support empty file name in getVirtualFileRef for serialized diagnostics

After https://reviews.llvm.org/D90484 libclang is unable to read a serialized diagnostic file
which contains a diagnostic which came from a file with an empty filename. The reason being is
that the serialized diagnostic reader is creating a virtual file for the "" filename, which now
fails after the changes in https://reviews.llvm.org/D90484. This patch restores the previous
behavior in getVirtualFileRef by allowing it to construct a file entry ref with an empty name by
pretending its name is "." so that the directory entry can be created.

Differential Revision: https://reviews.llvm.org/D100428