IntelliSense does not work if folder opened through a symbolic link

[Colengms]

This is derived from: #4569

Repro:

• On Linux, create a local directory. (i.e. 'mkdir actual')
• Create a source file (test.cpp) in that directory, with the following contents:

#if 0
int i;
#else
int j;
#endif

this should squiggle

• Create a symbolic link to that local directory. (i.e. 'ln -s actual softlink')
• Open the 'softlink' folder into VS Code.

Seeing: inactive regions are not dimmed. Squiggles are not applied.

Log output indicates that IntelliSense processing was successful against the file in the actual path, but the results are not applied. There are likely some comparisons of actual and softlink paths that are failing.

We're using realpath() to canonicalize paths on Linux and Mac, whereas on Win32 we don't try to resolve symbolic links at all. We may want to avoid using realpath(), and find an equivalent way to canonicalize paths that doesn't resolve symbolic links.