[cherry-pick][stable/20240723] [lldb][TypeSystemClang] Add warning and defensive checks when ASTContext is not fully initialized

[Michael137]

As this comment around target initialization implies:

  // This can be NULL if we don't know anything about the architecture or if
  // the target for an architecture isn't enabled in the llvm/clang that we
  // built

There are cases where we might fail to call InitBuiltinTypes when creating the backing ASTContext for a TypeSystemClang. If that happens, the builtins QualTypes, e.g., VoidPtrTy/IntTy/etc., are not initialized and dereferencing them as we do in GetBuiltinTypeForEncodingAndBitSize (and other places) will lead to nullptr-dereferences. Example backtrace:

(lldb) run
Assertion failed: (!isNull() && "Cannot retrieve a NULL type pointer"), function getCommonPtr, file Type.h, line 958.
Process 2680 stopped
* thread #15, name = '<lldb.process.internal-state(pid=2712)>', stop reason = hit program assert
    frame #4: 0x000000010cdf3cdc liblldb.20.0.0git.dylib`DWARFASTParserClang::ExtractIntFromFormValue(lldb_private::CompilerType const&, lldb_private::plugin::dwarf::DWARFFormValue const&) const (.cold.1) +
liblldb.20.0.0git.dylib`DWARFASTParserClang::ParseObjCMethod(lldb_private::ObjCLanguage::MethodName const&, lldb_private::plugin::dwarf::DWARFDIE const&, lldb_private::CompilerType, ParsedDWARFTypeAttributes
, bool) (.cold.1):
->  0x10cdf3cdc <+0>:  stp    x29, x30, [sp, #-0x10]!
    0x10cdf3ce0 <+4>:  mov    x29, sp
    0x10cdf3ce4 <+8>:  adrp   x0, 545
    0x10cdf3ce8 <+12>: add    x0, x0, #0xa25 ; "ParseObjCMethod"
Target 0: (lldb) stopped.
(lldb) bt
* thread #15, name = '<lldb.process.internal-state(pid=2712)>', stop reason = hit program assert
    frame #0: 0x0000000180d08600 libsystem_kernel.dylib`__pthread_kill + 8
    frame #1: 0x0000000180d40f50 libsystem_pthread.dylib`pthread_kill + 288
    frame #2: 0x0000000180c4d908 libsystem_c.dylib`abort + 128
    frame #3: 0x0000000180c4cc1c libsystem_c.dylib`__assert_rtn + 284
  * frame #4: 0x000000010cdf3cdc liblldb.20.0.0git.dylib`DWARFASTParserClang::ExtractIntFromFormValue(lldb_private::CompilerType const&, lldb_private::plugin::dwarf::DWARFFormValue const&) const (.cold.1) +
    frame #5: 0x0000000109d30acc liblldb.20.0.0git.dylib`lldb_private::TypeSystemClang::GetBuiltinTypeForEncodingAndBitSize(lldb::Encoding, unsigned long) + 1188
    frame #6: 0x0000000109aaaed4 liblldb.20.0.0git.dylib`DynamicLoaderMacOS::NotifyBreakpointHit(void*, lldb_private::StoppointCallbackContext*, unsigned long long, unsigned long long) + 384

This patch adds a one-time user-visible warning for when we fail to initialize the AST to indicate that initialization went wrong for the given target. Additionally, we add checks for whether one of the ASTContext QualTypes is invalid before dereferencing any builtin types.

The warning would look as follows:

(lldb) target create "a.out"
Current executable set to 'a.out' (arm64).
(lldb) b main
warning: Failed to initialize builtin ASTContext types for target 'some-unknown-triple'. Printing variables may behave unexpectedly.
Breakpoint 1: where = a.out`main + 8 at stepping.cpp:5:14, address = 0x0000000100003f90

rdar://134869779
(cherry picked from commit a5f3a2a)

[Michael137]

@swift-ci test

[Michael137]

@swift-ci test

[Michael137]

@swift-ci test

[Michael137]

@swift-ci test

[Michael137]

Windows failure is a Swift build failure. Unrelated to this PR