[modules] Retain the name as written for umbrella headers and directories, rather...

[modules] Retain the name as written for umbrella headers and directories, rather than converting to an absolute path. No observable change expected, but this allows us to correctly compute the module for an umbrella header, which later changes will require.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@237508 91177308-0d34-0410-b5e6-96231b3b80d8