Code style: don't patch expressions/statements with preprocessor

[elizarov]

This pull request establishes the following code style discipline: conditional directives are not allowed below the supported item boundary, such as inside an expression, declaration, or statement header.

Conditional compilation is allowed:

• When each branch contributes complete grammar items at the surrounding level: complete declarations, complete statements, field or method declarations, enum entries, macro definitions, or includes.
• For complete items in comma-separated lists, such as function arguments, braced initializer items, subscript items, declaration parameters, template arguments, template parameters, and enum entries.
• For conditional right-hand sides after = in assignment statements, alias declarations, and concept definitions.
• For conditional declaration-prefix modifiers.

This discipline keeps the code easier to read for humans and easier to parse, highlight, and format with tools. Code-analysis tools do not need to reason about partial expressions or statement headers across all conditional-compilation branches.

P.S. There's an ongoing work in https://github.com/elizarov/strictfmt to implement a rule-based source code formatter that correctly parses and formats all the source code when these restrictions are met.