[Cherry-pick into next] [lldb] Initialize a shared, blank SwiftASTContext in Objective-C frames

[adrian-prantl]

commit b5a1d34ec8a4a21da6bc0b169884f8e536aeb6dd
Author: Adrian Prantl <aprantl@apple.com>
Date:   Mon Jun 16 17:30:42 2025 -0700

    [lldb] Initialize a shared, blank SwiftASTContext in Objective-C frames
    
    The result of an Objective-C expression can be a Swift
    ValueObject. This means that `po foo` in Objective-C may run a Swift
    expression to print the object description of the result of the
    Objective-C expression `foo`. That expression has very few
    dependencies:
    
    ```
    Swift._DebuggerSupport.stringForPrintObject(Swift.unsafeBitCast(0x001234, to: AnyObject.self))
    ```
    
    To avoid scanning potentially incompatible compiler flags and/or doing
    implicit imports in anotherwise explicitly built project, this patch
    ensures that one SwiftASTContext per triple is created, with no other
    contextual information. This makes `po` in ObjC much faster and
    reliable, the downside of this is that
    
    ```expr -l Swift -- import ModuleName```
    
    will only work reliably for SDK modules outside of Swift frames.
    
    rdar://153332037