[clangd:vscode] Resolve symlinks for file paths from clangd.

[clangd:vscode] Resolve symlinks for file paths from clangd.

Summary:
For features like go-to-definition, clangd can point clients to symlink paths
(e.g. in bazel execroot) which might not be desired if the symlink points to a
file in the workspace. Clangd might not be able to build the file, and users
might prefer to edit the file on the real path.

This change converts file paths from clangd to real path (e.g. resolving symlinks).
Long term, we might want to the symlink handling logic to clangd where clangd
can better decide whether symlinks should be resolved according to e.g. compile
commands.

Reviewers: sammccall

Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D44158

llvm-svn: 327009