[ThinLTO] Default -enable-import-metadata to false

[MaskRay]

Summary

The default value is dependent on -DLLVM_ENABLE_ASSERTIONS={off,on} (D22167), which is
error-prone. The few tests checking !thinlto_src_module can specify -enable-import-metadata explicitly.

[tejohnson]

LGTM, I've been bitten by this in new tests as well. We can enable it when needed.

llvm/lib/Transforms/IPO/FunctionImport.cpp

 static cl::opt<bool> PrintImportFailures(
     "print-import-failures", cl::init(false), cl::Hidden,
     cl::desc("Print information for functions rejected for importing"));
 
 static cl::opt<bool> ComputeDead("compute-dead", cl::init(true), cl::Hidden,
                                  cl::desc("Compute dead symbols"));
 
 static cl::opt<bool> EnableImportMetadata(
-    "enable-import-metadata", cl::init(
-#if !defined(NDEBUG)
-                                  true /*Enabled with asserts.*/
-#else
-                                  false
-#endif
-                                  ),
-    cl::Hidden, cl::desc("Enable import metadata like 'thinlto_src_module'"));
+    "enable-import-metadata", cl::init(false), cl::Hidden,
+    cl::desc("Enable import metadata like 'thinlto_src_module'"));
 
 /// Summary file to use for function importing when using -function-import from
 /// the command line.
 static cl::opt<std::string>
     SummaryFile("summary-file",
                 cl::desc("The summary file to use for function importing."));
 
 /// Used when testing importing from distributed indexes via opt