[ThinLTO] Internalize readonly globals

An attempt to recommit r346584 after failure on OSX build bot.
Fixed cache key computation in ThinLTOCodeGenerator and added
test case

llvm-svn: 347033
diff --git a/llvm/test/Bitcode/summary_version.ll b/llvm/test/Bitcode/summary_version.ll
index b285da7..4913a1e 100644
--- a/llvm/test/Bitcode/summary_version.ll
+++ b/llvm/test/Bitcode/summary_version.ll
@@ -2,7 +2,7 @@
 ; RUN: opt  -module-summary  %s -o - | llvm-bcanalyzer -dump | FileCheck %s
 
 ; CHECK: <GLOBALVAL_SUMMARY_BLOCK
-; CHECK: <VERSION op0=4/>
+; CHECK: <VERSION op0=5/>