Add an index for Module Metadata record in the bitcode

Summary:
This index record the position for each metadata record in
the bitcode, so that the reader will be able to lazy-load
on demand each individual record.

We also make sure that every abbrev is emitted upfront so
that the block can be skipped while reading.

I don't plan to commit this before having the reader
counterpart, but I figured this can be reviewed mostly
independently.

Reviewers: pcc, tejohnson

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28083

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290684 91177308-0d34-0410-b5e6-96231b3b80d8

test/Bitcode/mdnodes-distinct-in-post-order.ll

@@ -17,6 +17,10 @@
 ; CHECK-NEXT:  <DISTINCT_NODE op0=1 op1=3 op2=2/>
 !4 = distinct !{!1, !3, !2}
 
+; Before the named records we emit the index containing the position of the
+; previously emitted records
+; CHECK-NEXT:   <INDEX {{.*}} (offset match)
+
 ; Note: named metadata nodes are not cannot reference null so their operands
 ; are numbered off-by-one.
 ; CHECK-NEXT:  <NAME

test/Bitcode/mdnodes-distinct-nodes-break-cycles.ll

@@ -22,6 +22,10 @@
 ; CHECK-NEXT:  <NODE op0=2/>
 !3 = !{!2}
 
+; Before the named records we emit the index containing the position of the
+; previously emitted records
+; CHECK-NEXT:   <INDEX {{.*}} (offset match)
+
 ; Note: named metadata nodes are not cannot reference null so their operands
 ; are numbered off-by-one.
 ; CHECK-NEXT:  <NAME

test/Bitcode/mdnodes-distinct-nodes-first.ll

@@ -11,6 +11,10 @@
 ; CHECK-NEXT:  <NODE op0=1/>
 !2 = !{!1}
 
+; Before the named records we emit the index containing the position of the
+; previously emitted records
+; CHECK-NEXT:   <INDEX {{.*}} (offset match)
+
 ; Note: named metadata nodes are not cannot reference null so their operands
 ; are numbered off-by-one.
 ; CHECK-NEXT:  <NAME

test/Bitcode/mdnodes-in-post-order.ll

@@ -15,6 +15,9 @@
 ; CHECK-NEXT:    'leaf
 ; CHECK-NEXT:  }
 
+; Before the records we emit an offset to the index for the block
+; CHECK-NEXT:   <INDEX_OFFSET
+
 ; The leafs should come first (in either order).
 ; CHECK-NEXT:  <NODE op0=1/>
 ; CHECK-NEXT:  <NODE op0=2/>
@@ -27,6 +30,10 @@
 ; CHECK-NEXT:  <NODE op0=3 op1=5 op2=4/>
 !6 = !{!3, !5, !4}
 
+; Before the named records we emit the index containing the position of the
+; previously emitted records
+; CHECK-NEXT:   <INDEX {{.*}} (offset match)
+
 ; Note: named metadata nodes are not cannot reference null so their operands
 ; are numbered off-by-one.
 ; CHECK-NEXT:  <NAME

test/Bitcode/metadata-function-blocks.ll

@@ -19,6 +19,9 @@
 ; Each node gets a new number.  Bottom-up traversal of nodes.
 !named = !{!6}
 
+; Before the records we emit an offset to the index for the block
+; CHECK-NEXT:   <INDEX_OFFSET
+
 ; CHECK-NEXT:   <NODE op0=1/>
 !4 = !{!"named"}