[lldb] Don't crash when the ASTImporter produces diagnostics but instead log them.

When playing with the C++ module prototype I noticed I can get LLDB to crash
by making a result type that depends on __make_integer_seq (a BuiltinTemplate)
which is not supported by the ASTImporter yet. This causes the ASTImporter to emit
a diagnostic when copying the type to the ScratchASTContext. As deporting the result
type is done after we are done parsing and the Clang's diagnostic engine asserts that
it can only be used during parsing, it crashes LLDB while trying to render the diagnostic
in the HandleDiagnostic method of ClangDiagnosticManagerAdapter.

This patch just moves the HandleDiagnostic call to Clang behind our check that we still
have a DiagnosticManager (which we remove after parsing) which prevents the assert
from firing. We also shouldn't ignore such diagnostics, so I added a log statement for
them.

There doesn't seem to way to test this as these diagnostic only happen when we copy
a node that's not supported by the ASTImporter which should never happen once
we can copy everything with the ASTImporter, so every test case we add here will
eventually become invalid.

(Note that most of this diff is just whitespace changes as we now use an early exit
instead of a huge 'if' block).

llvm-svn: 374145