[lldb][ClangASTImporter][NFC] Emit a log message when we break MapImported invariant (#112748)

This patch emits a warning into the expression log when we call
`MapImported` on a decl which has already been imported, but with a new
`to` destination decl. In asserts builds this would lead to triggering
this [ASTImporter::MapImported
assertion](https://github.com/llvm/llvm-project/blob/6d7712a70c163d2ae9e1dc928db31fcb45d9e404/clang/lib/AST/ASTImporter.cpp#L10493-L10494).
In no-asserts builds we will likely crash, in potentially non-obvious
ways. The hope is that the log message will help in diagnosing this type
of issue in the field.

The underlying issue is discussed in more detail in:
https://github.com/llvm/llvm-project/pull/112566.

In a non-asserts build, the last few expression log entries would look
as follows:
```
     CompleteTagDecl on (ASTContext*)scratch ASTContext Completing (TagDecl*)0x00000001132d31d0 named Foo
       CTD Before:
CXXRecordDecl 0x1132d31d0 <<invalid sloc>> <invalid sloc> <undeserialized declarations> struct Foo

 [ClangASTImporter] WARNING: overwriting an already imported decl '0x000000014378fd80' ('Foo') from '0x0000000143790c00' with 0x00000001132d31d0. Likely due to a name conflict when importing 'Foo'.
     [ClangASTImporter] Imported (FieldDecl*)0x0000000143790220, named service (from (Decl*)0x0000000143791270), metadata 271
     [ClangASTImporter] Decl has no origin information in (ASTContext*)0x00000001132c8c00
 FindExternalLexicalDecls on (ASTContext*)0x0000000143c1f600 'scratch ASTContext' in 'Foo' (CXXRecordDecl*)0x000000014378FD80
   FELD Original decl (ASTContext*)0x00000001132c8c00 (Decl*)0x0000000143790c00:
CXXRecordDecl 0x143790c00 <<invalid sloc>> <invalid sloc> struct Foo definition
|-DefinitionData pass_in_registers aggregate standard_layout trivially_copyable pod trivial literal
| |-DefaultConstructor exists trivial needs_implicit
| |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param
| |-MoveConstructor exists simple trivial needs_implicit
| |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param
| |-MoveAssignment exists simple trivial needs_implicit
| `-Destructor simple irrelevant trivial needs_implicit
|-FieldDecl 0x143791270 <<invalid sloc>> <invalid sloc> service 'Service *'
`-FieldDecl 0x1437912c8 <<invalid sloc>> <invalid sloc> mach_endpoint 'int'

   FELD Adding [to CXXRecordDecl Foo] lexical FieldDecl FieldDecl 0x143791270 <<invalid sloc>> <invalid sloc> service 'Service *'

   FELD Adding [to CXXRecordDecl Foo] lexical FieldDecl FieldDecl 0x1437912c8 <<invalid sloc>> <invalid sloc> mach_endpoint 'int'

     [ClangASTImporter] Imported (FieldDecl*)0x0000000143790278, named mach_endpoint (from (Decl*)0x00000001437912c8), metadata 280
     [ClangASTImporter] Decl has no origin information in (ASTContext*)0x00000001132c8c00
```
Note how we start "completing" `Foo`. Then emit our new `WARNING`.
Shortly after, we crash, and the log abruptly ends.

rdar://135551810

0 files changed, 0 insertions, 0 deletions