[clang-tools-extra] [clang-doc] Add a top-level index.html page (PR #171899)

[Paul Kirth via cfe-commits]

================
@@ -1,7 +1,7 @@
 // RUN: rm -rf %t && mkdir -p %t
 // RUN: clang-doc --format=html --output=%t --executor=standalone %s
 // RUN: clang-doc --format=md --output=%t --executor=standalone %s
-// RUN: FileCheck %s < %t/@nonymous_namespace/AnonClass.md -check-prefix=MD-ANON-CLASS-LINE
+// run: filecheck %s < %t/@nonymous_namespace/anonclass.md -check-prefix=md-anon-class-line
----------------
ilovepi wrote:

```suggestion
// RUN: FileCheck %s < %t/@nonymous_namespace/anonclass.md -check-prefix=MD-ANON-CLASS-LINE
```
Seems like you may have not meant to lowercase all of these? I'm unsure on the filename, but I'd be surprised about any of the rest, since I'd think the checks wouldn't run anymore.

https://github.com/llvm/llvm-project/pull/171899