[Expression parser] Setting to enable use of ExternalASTMerger

This setting can be enabled like this at the target level:

(lldb) settings set target.experimental.use-modern-type-lookup true

This causes several new behaviors in the Clang expression parser:

- It completely disables use of ClangASTImporter.  None are created
  at all, and all users of it are now conditionalized on its
  presence.

- It instead constructs a per-expression ExternalASTMerger, which
  exists inside Clang and contains much of the type completion
  logic that hitherto lived in ExternalASTSource,
  ClangExpressionDeclMap, and ClangASTImporter.

- The expression parser uses this Merger as a backend for copying
  and completing types.

- It also constructs a persistent ExternalASTMerger which is
  connected to the Target's persistent AST context.

This is a major chunk of LLDB functionality moved into Clang.  It
can be tested in two ways:

1. For an individual debug session, enable the setting before
   running a target.

2. For the testsuite, change the option to be default-true.  This
   is done in Target.cpp's g_experimental_properties.  The
   testsuite is not yet clean with this, so I have not committed
   that switch.

I have filed a Bugzilla for extending the testsuite to allow
custom settings for all tests:
  https://bugs.llvm.org/show_bug.cgi?id=34771

I have also filed a Bugzilla for fixing the remaining testsuite
failures with this setting enabled:
  https://bugs.llvm.org/show_bug.cgi?id=34772

llvm-svn: 314458