[modules] Accept equivalent module caches from different symlink (#90925)

Use `VFS.equivalent()`, which follows symlinks, to check if two module
cache paths are equivalent. This prevents a PCH error when building from
a different path that is a symlink of the original.

```
error: PCH was compiled with module cache path '/home/foo/blah/ModuleCache/2IBP1TNT8OR8D', but the path is currently '/data/users/foo/blah/ModuleCache/2IBP1TNT8OR8D'
1 error generated.
```

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Support/VirtualFileSystem.cpp b/llvm/lib/Support/VirtualFileSystem.cpp
index 152fcfe..54b9c38 100644
--- a/llvm/lib/Support/VirtualFileSystem.cpp
+++ b/llvm/lib/Support/VirtualFileSystem.cpp
@@ -151,6 +151,16 @@ bool FileSystem::exists(const Twine &Path) {
   return Status && Status->exists();
 }
 
+llvm::ErrorOr<bool> FileSystem::equivalent(const Twine &A, const Twine &B) {
+  auto StatusA = status(A);
+  if (!StatusA)
+    return StatusA.getError();
+  auto StatusB = status(B);
+  if (!StatusB)
+    return StatusB.getError();
+  return StatusA->equivalent(*StatusB);
+}
+
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 void FileSystem::dump() const { print(dbgs(), PrintType::RecursiveContents); }
 #endif