[ThinLTO] Parse module summary index from assembly

Summary:
Adds assembly parsing support for the module summary index (follow on
to r333335 which added the assembly writing support).

I added support to llvm-as to invoke the index parsing, so that it can
create either a bitcode file with a Module and a per-module index, or
a combined index without a Module.

I will send follow on patches soon to do the following:
- add support to tools such as llvm-lto2 to parse the per-module indexes
from assembly instead of bitcode when testing the thin link.
- verification support.

Depends on D47844 and D47842.

Reviewers: pcc, dexonsmith, mehdi_amini

Subscribers: inglorion, eraman, steven_wu, llvm-commits

Differential Revision: https://reviews.llvm.org/D47905

llvm-svn: 335602

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -197,7 +197,7 @@ public:
           // otherwise we would have a Value for it). If so, synthesize
           // a value id.
           for (auto &CallEdge : FS->calls())
-            if (!CallEdge.first.getValue())
+            if (!CallEdge.first.haveGVs() || !CallEdge.first.getValue())
               assignValueId(CallEdge.first.getGUID());
   }
 
@@ -230,7 +230,7 @@ private:
 
   // Helper to get the valueId for the type of value recorded in VI.
   unsigned getValueId(ValueInfo VI) {
-    if (!VI.getValue())
+    if (!VI.haveGVs() || !VI.getValue())
       return getValueId(VI.getGUID());
     return VE.getValueID(VI.getValue());
   }