ASTGen always builds with the host Swift compiler, without requiring
bootstrapping, and is enabled in more places. Move the regex literal
parsing logic there so it is enabled in more host environments, and
makes use of CMake's Swift support. Enable all of the regex literal
tests when ASTGen is built, to ensure everything is working.
Remove the "AST" and "Parse" Swift modules from SwiftCompilerSources,
because they are no longer needed.
* 'ASTGenVisitor' has a reference to a legacy C++ Parser configured for
ASTGen.
* If 'ASTGenVisitor' encounters a AST node that hasn't been migrated,
call parse(Decl|Stmt|Expr|Type) to parse the position using the legacy
parser.
* The legacy parser calls ASTGen's
'swift_ASTGen_build(Decl|Stmt|Expr|Type)' for each ASTNode "parsing"
(unless the call is not directly from the ASTGen.)
rdar://117151886
Merge with BasicBridging and ASTBridging
respectively. The changes here should be pretty
uncontroversial, I tried to keep it to just moving
code about.