[clang] [clang] Improve CI output when trailing whitespace is found (PR #66649)

[Louis Dionne via cfe-commits]

ldionne wrote:

@asl This should make it a bit more explicit. I see this now when running locally:

```
+ echo '*** Checking for trailing whitespace left in Clang source files ***'
*** Checking for trailing whitespace left in Clang source files ***
+ grep -rnI '[[:blank:]]$' clang/lib clang/include clang/docs
clang/include/clang-c/Index.h:38:
+ echo '*** Trailing whitespace has been found in Clang source files as described above ***'
*** Trailing whitespace has been found in Clang source files as described above ***
+ exit 1
```

WDYT? It's still a bit cluttered by the fact that we run with `set -x` but I think it might be good enough.

https://github.com/llvm/llvm-project/pull/66649