[Darwin] Export new weak external symbols when compiling with coverage

Some weak external symbols were added to the profile runtime in D49953,
and on Darwin, these need to be exported for tapi verification purposes.

I've tightened the test so that future breakages can be caught earlier.

rdar://45831054

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@346276 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit e7b027c)

lib/Driver/ToolChains/Darwin.cpp

@@ -1029,6 +1029,8 @@ void Darwin::addProfileRTLibs(const ArgList &Args,
     addExportedSymbol(CmdArgs, "___llvm_profile_filename");
     addExportedSymbol(CmdArgs, "___llvm_profile_raw_version");
     addExportedSymbol(CmdArgs, "_lprofCurFilename");
+    addExportedSymbol(CmdArgs, "_lprofDirMode");
+    addExportedSymbol(CmdArgs, "_lprofMergeValueProfData");
   }
 }