[lldb] Pass a valid SourceLocation to Sema::RequireCompleteType in ASTResultSynthesizer

Sema::RequireCompleteTypeImpl is supposed to have an assert that checks that the
SourceLocation that is passed in is always valid. It's currently commented out, but
as soon as this assert goes active, nearly every LLDB expression will start crashing as
we always pass in an invalid SourceLocation from the ASTResultSynthesizer.

This patch just passes in the valid SourceLocation of the expression (which is
the SourceLocation where the complete type is required) to prevent that from happening.