[LTO] Record whether LTOUnit splitting is enabled in index

Summary:
Records in the module summary index whether the bitcode was compiled
with the option necessary to enable splitting the LTO unit
(e.g. -fsanitize=cfi, -fwhole-program-vtables, or -fsplit-lto-unit).

The information is passed down to the ModuleSummaryIndex builder via a
new module flag "EnableSplitLTOUnit", which is propagated onto a flag
on the summary index.

This is then used during the LTO link to check whether all linked
summaries were built with the same value of this flag. If not, an error
is issued when we detect a situation requiring whole program visibility
of the class hierarchy. This is the case when both of the following
conditions are met:
1) We are performing LowerTypeTests or Whole Program Devirtualization.
2) There are type tests or type checked loads in the code.

Note I have also changed the ThinLTOBitcodeWriter to also gate the
module splitting on the value of this flag.

Reviewers: pcc

Subscribers: ormris, mehdi_amini, Prazek, inglorion, eraman, steven_wu, dexonsmith, arphaman, dang, llvm-commits

Differential Revision: https://reviews.llvm.org/D53890

llvm-svn: 350948

test/Bitcode/thinlto-function-summary-callgraph-sample-profile-summary.ll

@@ -29,6 +29,7 @@
 
 ; CHECK-LABEL:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
+; CHECK-NEXT:    <FLAGS
 ; CHECK-NEXT:    <VALUE_GUID op0=26 op1=123/>
 ; op4=none1 op6=hot1 op8=cold1 op10=none2 op12=hot2 op14=cold2 op16=none3 op18=hot3 op20=cold3 op22=123
 ; CHECK-NEXT:    <PERMODULE_PROFILE {{.*}} op6=7 op7=0 op8=1 op9=3 op10=4 op11=1 op12=8 op13=0 op14=2 op15=3 op16=5 op17=1 op18=9 op19=0 op20=3 op21=3 op22=6 op23=1 op24=26 op25=4/>