[ThinLTO]Record import type in GlobalValueSummary::GVFlags (#87597)

The motivating use case is to support import the function declaration
across modules to construct call graph edges for indirect calls [1]
when importing the function definition costs too much compile time
(e.g., the function is too large has no `noinline` attribute).
1. Currently, when the compiled IR module doesn't have a function
definition but its postlink combined summary contains the function
summary or a global alias summary with this function as aliasee, the
function definition will be imported from source module by IRMover. The
implementation is in FunctionImporter::importFunctions [2]
2. In order for FunctionImporter to import a declaration of a function,
both function summary and alias summary need to carry the def / decl
state. Specifically, all existing summary fields doesn't differ across
import modules, but the def / decl state of is decided by
`<ImportModule, Function>`.

This change encodes the def/decl state in `GlobalValueSummary::GVFlags`.

In the subsequent changes
1. The indexing step `computeImportForModule` [3]
will compute the set of definitions and the set of declarations for each
module, and passing on the information to bitcode writer.
2. Bitcode writer will look up the def/decl state and sets the state
when it writes out the flag value. This is demonstrated in
https://github.com/llvm/llvm-project/pull/87600
3. Function importer will read the def/decl state when reading the
combined summary to figure out two sets of global values, and IRMover
will be updated to import the declaration (aka linkGlobalValuePrototype [4])
into the destination module.

- The next change is https://github.com/llvm/llvm-project/pull/87600

[1] mentioned in rfc https://discourse.llvm.org/t/rfc-for-better-call-graph-sort-build-a-more-complete-call-graph-by-adding-more-indirect-call-edges/74029#support-cross-module-function-declaration-import-5
[2] https://github.com/llvm/llvm-project/blob/3b337242ee165554f0017b00671381ec5b1ba855/llvm/lib/Transforms/IPO/FunctionImport.cpp#L1608-L1764
[3] https://github.com/llvm/llvm-project/blob/3b337242ee165554f0017b00671381ec5b1ba855/llvm/lib/Transforms/IPO/FunctionImport.cpp#L856
[4] https://github.com/llvm/llvm-project/blob/3b337242ee165554f0017b00671381ec5b1ba855/llvm/lib/Linker/IRMover.cpp#L605

2 files changed, 13 insertions, 0 deletions

diff --git a/llvm/test/Transforms/WholeProgramDevirt/Inputs/import-indir.yaml b/llvm/test/Transforms/WholeProgramDevirt/Inputs/import-indir.yaml
index 30159c5..22533ed 100644
--- a/llvm/test/Transforms/WholeProgramDevirt/Inputs/import-indir.yaml
+++ b/llvm/test/Transforms/WholeProgramDevirt/Inputs/import-indir.yaml
@@ -2,6 +2,7 @@
 GlobalValueMap:
   42:
     - Live: true
+      ImportType: 0
       TypeTestAssumeVCalls:
         - GUID: 123
           Offset: 0
@@ -22,6 +23,9 @@ GlobalValueMap:
             GUID: 456
             Offset: 8
           Args: [24, 12]
+  43:
+    - Live: true
+      ImportType : 1
 TypeIdMap:
   typeid1:
     WPDRes:
diff --git a/llvm/test/Transforms/WholeProgramDevirt/import-indir.ll b/llvm/test/Transforms/WholeProgramDevirt/import-indir.ll
index 1d74a59..e4d6f1d 100644
--- a/llvm/test/Transforms/WholeProgramDevirt/import-indir.ll
+++ b/llvm/test/Transforms/WholeProgramDevirt/import-indir.ll
@@ -10,6 +10,7 @@
 ; SUMMARY-NEXT:      Live:                true
 ; SUMMARY-NEXT:      Local:               false
 ; SUMMARY-NEXT:      CanAutoHide:         false
+; SUMMARY-NEXT:      ImportType:          0
 ; SUMMARY-NEXT:      TypeTestAssumeVCalls:
 ; SUMMARY-NEXT:        - GUID:            123
 ; SUMMARY-NEXT:          Offset:          0
@@ -30,6 +31,14 @@
 ; SUMMARY-NEXT:            GUID:            456
 ; SUMMARY-NEXT:            Offset:          8
 ; SUMMARY-NEXT:          Args: [ 24, 12 ]
+; SUMMARY-NEXT: 43:
+; SUMMARY-NEXT:   - Linkage:                0
+; SUMMARY-NEXT:     Visibility:             0
+; SUMMARY-NEXT:     NotEligibleToImport:   false
+; SUMMARY-NEXT:     Live:                  true
+; SUMMARY-NEXT:     Local:                 false
+; SUMMARY-NEXT:     CanAutoHide:           false
+; SUMMARY-NEXT:     ImportType:            1
 ; SUMMARY-NEXT: TypeIdMap:
 ; SUMMARY-NEXT:   typeid1:
 ; SUMMARY-NEXT:     TTRes: