[llvm] [Support] Better error msg when cache dir can't be created. (PR #69575)

[Fangrui Song via llvm-commits]

================
@@ -0,0 +1,20 @@
+; REQUIRES: x86
----------------
MaskRay wrote:

https://maskray.me/blog/2021-08-08-toolchain-testing#the-test-checks-at-the-wrong-layer

A better test is llvm-lto2 --cache-dir in `llvm/test/ThinLTO/X86`

https://github.com/llvm/llvm-project/pull/69575