Currently, normal globals are represented as a PatternBindingDecl and a VarDecl in the AST, directly under the SourceFile:
```
// var variable_name = 42, compiled with -parse-as-library
(source_file ...
(pattern_binding_decl ...
(pattern_entry ...
(pattern_named ... "variable_name") ...
(var_decl "variable_name" ...
```
Top-level globals are represented more like local variables, under a TopLevelCodeDecl. Note that the VarDecl is still at the file scope. In SILGen, this case has some special handling to use the a storage of a global variable, and to avoid cleanups (see `emitInitializationForVarDecl`). Effectively, this means the globals are initialized inside the `main` function.
```
// var variable_name = 42, compiled without -parse-as-library
(source_file ...
(top_level_code_decl ...
(brace_stmt ...
(pattern_binding_decl ...
(pattern_named ... "variable_name") ...
(var_decl "variable_name" ... top_level_global
```
SE-0492 needs top-level globals that have a `@section` annotation to behave like a normal global -- initialization must happen statically, and not in `main`. This PR changes the parsing of those globals to match normal globals, without the TopLevelCodeDecl wrapper. SILGen and IRGen then handles them correctly.
Removes the underscored prefixes from the @_section and @_used attributes, making them public as @section and @used respectively. The SymbolLinkageMarkers experimental feature has been removed as these attributes are now part of the standard language. Implemented expression syntactic checking rules per SE-0492.
Major parts:
- Renamed @_section to @section and @_used to @used
- Removed the SymbolLinkageMarkers experimental feature
- Added parsing support for the old underscored names with deprecation warnings
- Updated all tests and examples to use the new attribute names
- Added syntactic validation for @section to align with SE-0492 (reusing the legality checker by @artemcm)
- Changed @DebugDescription macro to explicitly use a tuple type instead of type inferring it, to comply with the expression syntax rules
- Added a testcase for the various allowed and disallowed syntactic forms, `test/ConstValues/SectionSyntactic.swift`.
Syntactically verify that initializer expressions of '@const' variables and argument expressions to '@const' parameters consist strictly of syntactically-verifiable set of basic values and operations
* `sitofp` signed integer to floating point
* `rint` round floating point to integral
* `bitcast` between integer and floating point
Constant folding `bitcast`s also made it necessary to rewrite constant folding for Nan and inf values, because the old code explicitly checked for `bitcast` intrinsics.
Relying on constant folded `bitcast`s makes the new version much simpler.
It is important to constant fold these intrinsics already in SIL because it enables other optimizations.
