[lldb][ClangExpression] Fix LLDB_LOG incorrect format specifier

Previously this would log:
```
 FindExternalLexicalDecls on (ASTContext*)0x00000005CE825200 'Expression
ASTContext for '<user expression 0>'' in 'weak_ptr'
(%sDecl*)ClassTemplateSpecialization
 FindExternalLexicalDecls on (ASTContext*)0x00000005CE825200 'Expression
ASTContext for '<user expression 0>'' in '__shared_count'
(%sDecl*)CXXRecord
```

Note that the `%s` isn't actually respected. This patch fixes this
by providing the format specifiers that `lldb::formatv` supports.

Differential Revision: https://reviews.llvm.org/D132709