[ThinLTO] Renaming of function index to module summary index (NFC)

With the changes in r263275, there are now more than just functions in
the summary. Completed the renaming of data structures (started in
r263275) to reflect the wider scope. In particular, changed the
FunctionIndex* data structures to ModuleIndex*, and renamed related
variables and comments. Also renamed the files to reflect the changes.

A companion clang patch will immediately succeed this patch to reflect
this renaming.

llvm-svn: 263490
diff --git a/llvm/test/Bitcode/thinlto-function-summary-refgraph.ll b/llvm/test/Bitcode/thinlto-function-summary-refgraph.ll
index 5f9fc3b..d8698bf 100644
--- a/llvm/test/Bitcode/thinlto-function-summary-refgraph.ll
+++ b/llvm/test/Bitcode/thinlto-function-summary-refgraph.ll
@@ -1,5 +1,5 @@
 ; Test to check both the callgraph and refgraph in summary
-; RUN: llvm-as -function-summary %s -o %t.o
+; RUN: llvm-as -module-summary %s -o %t.o
 ; RUN: llvm-bcanalyzer -dump %t.o | FileCheck %s
 
 ; See if the calls and other references are recorded properly using the