The ClangASTSource has a lock that globally disables all lookups into the external AST source
when we explicitly "guarded" copy a type. It's not used for anything else, so importing declarations
or importing types that are dependencies of a declaration actually won't activate that lock. The
lookups it is supposed to prevent also don't actually happen in our test suite. The check in
ClangExpressionDeclMap::FindExternalVisibleDecls is never executed and the check in
the ClangASTSource::FindExternalVisibleDeclsByName is only ever reached by the
Import-std-module tests (which explicitly do a lookup into the expression context on purpose).
This lock was added in 6abfabff6158076eccdf6fcac5a12894039de2c9 as a replacement for a list
of types we already looked up which appeared to be an optimisation strategy. I assume back then
this lock had a purpose but these days the ASTImporter and LLDB seem to be smart enough to
avoid whatever lookups this tried to prevent.
I would say we remove it from LLDB. The main reason is that it blocks D81561 (which explicitly
does a specific lookup to resolve placeholder types produced by -flimit-debug-info) but it's
semantics are also very confusing. The naming implies it's a flag to indicate when we import
something at the moment which is practically never true as described above. Also the fact
that it makes our ExternalASTSource alternate between doing lookups into the debug info
and pretending it doesn't know any external decls could really break our lookup in some weird way
if Clang decides to cache a fake empty lookup result that was generated while the lock was active.