[lld][COFF] Find libraries with relative paths. r=hans,maskray,mstorsjo,phosek,rnk

[thieta]

Summary

This patch is spun out of https://reviews.llvm.org/D151188
and makes it possible for lld-link to find libraries with
relative paths. This will be used later to implement the
changes to autolinking runtimes explained in #63827

[hans]

lgtm

lld/docs/ReleaseNotes.rst

40

Perhaps a short example could be added here to make it more clear what changed.

[thieta]

Add example to release notes

[hans]

lld/docs/ReleaseNotes.rst

41

Technically, relative/path/my.lib would already work if that's relative to the current working directory. What's new is that it will now also work relative to /libpaths.

[thieta]

Clarify release note even further

[mstorsjo]

LGTM, thanks!

lld/COFF/Driver.cpp

 StringRef LinkerDriver::findFile(StringRef filename) {
   auto getFilename = [this](StringRef filename) -> StringRef {
     if (ctx.config.vfs)
       if (auto statOrErr = ctx.config.vfs->status(filename))
         return saver().save(statOrErr->getName());
     return filename;
   };
 
-  bool hasPathSep = (filename.find_first_of("/\\") != StringRef::npos);
-  if (hasPathSep)
+  if (sys::path::is_absolute(filename))
     return getFilename(filename);
   bool hasExt = filename.contains('.');
   for (StringRef dir : searchPaths) {
     SmallString<128> path = dir;
     sys::path::append(path, filename);
     path = SmallString<128>{getFilename(path.str())};
     if (sys::fs::exists(path.str()))
       return saver().save(path.str());

lld/docs/ReleaseNotes.rst

* ``PT_RISCV_ATTRIBUTES`` is added to include the SHT_RISCV_ATTRIBUTES section.
   (`D152065 <https://reviews.llvm.org/D152065>`_)
 
 Breaking changes
 ----------------
 
 COFF Improvements
 -----------------
 
+* lld-link can now find libraries with relative paths that are relative to
+  `/libpath`. Before it would only be able to find libraries relative to the
+  current directory.
+  I.e. ``lld-link /libpath:c:\relative\root relative\path\my.lib`` where before
+  we would have to do ``lld-link /libpath:c:\relative\root\relative\path my.lib``
+
 MinGW Improvements
 ------------------
 
 MachO Improvements
 ------------------
 
 WebAssembly Improvements
 ------------------------
 
 Fixes
 #####
 
 * Arm exception index tables (.ARM.exidx sections) are now output
   correctly when they are at a non zero offset within their output
   section. (`D148033 <https://reviews.llvm.org/D148033>`_)

lld/test/COFF/relative_search_paths.test

+We should be able to find libraries with relative search paths.
+# RUN: mkdir -p %t.dir/relative/path
+# RUN: cp %p/Inputs/std64.lib %t.dir/relative/path
+# RUN: lld-link %p/Inputs/hello64.obj /libpath:%t.dir relative/path/std64.lib /entry:main