[clang][FileManager] fillRealPathName even if we aren't opening the file

The pathname wasn't previously filled when the getFile() method was called with openFile = false.
We are caching FileEntry-s in ParsedAST::Includes in clangd and this caused the problem.

This fixes an internal test failure in clangd - ClangdTests.GoToInclude.All

rdar://47536127

Differential Revision: https://reviews.llvm.org/D58213

llvm-svn: 354075