[clang-tools-extra] [clangd] Allow specifying what headers are always included via "" or <> (PR #67749)

[kleines Filmröllchen via cfe-commits]

kleinesfilmroellchen wrote:

I investigated the test failures some more in the context of where the bug affects real uses and I'm unsure as to what's going on.
```yaml
Style:
  QuotedHeaders: ["blahblahblah"]
  AngledHeaders: ["AK/.*", "Userland/.*", "Kernel/.*", "Applications/.*", "Lib.*/.*"]
```
will match all regexes correctly.
```yaml
Style:
  AngledHeaders: ["AK/.*", "Userland/.*", "Kernel/.*", "Applications/.*", "Lib.*/.*"]
```
will make AngledHeaders completely disappear from the parsed and compiled config; apparently the dictionary key doesn't even exist anymore. Exactly this behavior makes a test fail on CI.
```yaml
Style:
  QuotedHeaders: ["blahblahblah"]
```
will keep QuotedHeaders in the parsed and compiled config and works as expected.
There is no apparent difference in how these two keys are handled, I have cross-checked my config parser and compiler code with existing code countless times. Really no clue what's happening here other than QuotedHeaders apparently needs to exist or AngledHeaders vanishes.
CC @HighCommander4 

https://github.com/llvm/llvm-project/pull/67749