[clang] [clang-format] Add MainIncludeChar option. (PR #78752)

[Owen Pan via cfe-commits]

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+// Test the combination of regrouped include directives, via regexes and
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owenca wrote:

> Ok I will move them there, thanks :) Should I remove these lit tests or are they complementary?

Please remove them from the lit tests.

https://github.com/llvm/llvm-project/pull/78752