[ThinLTO]Supports declaration import for global variables in distributed ThinLTO (#117616)

When `-import-declaration` option is enabled, declaration import is
supported for functions. https://github.com/llvm/llvm-project/pull/88024
has the context for this option.

This patch supports declaration import for global variables in
distributed ThinLTO. The motivating use case is to propagate `dso_local`
attribute of global variables across modules, to optimize global
variable access when a binary is built with
`-fno-direct-access-external-data`.
* With `-fdirect-access-external-data`, non thread-local global
variables will [have `dso_local`
attributes](https://github.com/llvm/llvm-project/blob/fe3c23b439b9a2d00442d9bc6a4ca86f73066a3d/clang/lib/CodeGen/CodeGenModule.cpp#L1730-L1746).
This optimizes the global variable access as shown by
https://gcc.godbolt.org/z/vMzWcKdh3

1 files changed, 2 insertions, 1 deletions

diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index 63f4e34..0444cb9 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -4900,7 +4900,8 @@ void IndexBitcodeWriter::writeCombinedGlobalValueSummary() {
       NameVals.push_back(*ValueId);
       assert(ModuleIdMap.count(VS->modulePath()));
       NameVals.push_back(ModuleIdMap[VS->modulePath()]);
-      NameVals.push_back(getEncodedGVSummaryFlags(VS->flags()));
+      NameVals.push_back(
+          getEncodedGVSummaryFlags(VS->flags(), shouldImportValueAsDecl(VS)));
       NameVals.push_back(getEncodedGVarFlags(VS->varflags()));
       for (auto &RI : VS->refs()) {
         auto RefValueId = getValueId(RI.getGUID());