[Support] Better error msg when cache dir can't be created.

lld/test/COFF/lto-cache-errors.ll

@@ -0,0 +1,20 @@
+; REQUIRES: x86
+;; Not supported on windows since we use permissions to deny the creation
+; UNSUPPORTED: system-windows
+
+; RUN: opt -module-hash -module-summary %s -o %t.o
+; RUN: opt -module-hash -module-summary %p/Inputs/lto-cache.ll -o %t2.o
+; RUN: rm -Rf %t.cache && mkdir %t.cache
+; RUN: chmod 444 %t.cache
+
+;; Check emit warnings when we can't create the cache dir
+; RUN: not --crash lld-link /lldltocache:%t.cache/nonexistant/ /out:%t3 /entry:main %t2.o %t.o 2>&1 | FileCheck %s
+; CHECK: LLVM ERROR: can't create cache directory {{.*}}/nonexistant/: Permission denied
+
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+define void @globalfunc() #0 {
+entry:
+  ret void
+}

llvm/lib/Support/Caching.cpp

@@ -145,7 +145,9 @@ Expected<FileCache> llvm::localCache(const Twine &CacheNameRef,
       // ensures the filesystem isn't mutated until the cache is.
       if (std::error_code EC = sys::fs::create_directories(
               CacheDirectoryPath, /*IgnoreExisting=*/true))
-        return errorCodeToError(EC);
+        return createStringError(EC, Twine("can't create cache directory ") +
+                                         CacheDirectoryPath + ": " +
+                                         EC.message());
 
       // Write to a temporary to avoid race condition
       SmallString<64> TempFilenameModel;