[clang-format] IncludeCategories: Add a regex group named main into the Regex Solver

[Febbe]

There are many issues, regarding main file sorting.
Also, we have the issue, that a main source file can have 2 main headers, e.g. a.cpp a.h and a_priv.h

With IncludeIsMainRegex and IncludeIsMainSourceRegex only a mapping between a header to a source can be defined, but not the other way round.

A solution to all issues would be, to add a parameter in the form of a named capture group to the regex.
For example, a hypothetical group definition (?<main>regex) would map to \k<main>, which then maps to the main file's stem.

IncludeCategories:
  - Regex:           '\k<main>_priv\.h"$'
    Priority:        0

The above would then match a_priv.h as expected.
With that, programmers could also override the default behavior, to accomplish:

#58284
#37620 #39735 #53013

Possible implementation could be:

• generate all possible main-header stems from IncludeIsMainRegex and IncludeIsMainSourceRegex
• supply all (including the original stem) to the Regex solver as group (do this in a loop, if it's not possible to accomplish this via one group)

[llvmbot]

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