[debuginfo][coro] Emit debug info labels for coroutine resume points (#141937)

RFC on discourse:
https://discourse.llvm.org/t/rfc-debug-info-for-coroutine-suspension-locations-take-2/86606

With this commit, we add `DILabel` debug infos to the resume points of a
coroutine. Those labels can be used by debugging scripts to figure out
the exact line and column at which a coroutine was suspended by looking
up current `__coro_index` value inside the coroutines frame, and then
searching for the corresponding label inside the coroutine's resume
function.

The DWARF information generated for such a label looks like:

```
0x00000f71:     DW_TAG_label
                  DW_AT_name    ("__coro_resume_1")
                  DW_AT_decl_file       ("generator-example.cpp")
                  DW_AT_decl_line       (5)
                  DW_AT_decl_column     (3)
                  DW_AT_artificial      (true)
                  DW_AT_LLVM_coro_suspend_idx   (0x01)
                  DW_AT_low_pc  (0x00000000000019be)
```

The labels can be mapped to their corresponding `__coro_idx` values
either via their naming convention `__coro_resume_<N>` or using the new
`DW_AT_LLVM_coro_suspend_idx` attribute. In gdb, those line numebrs can
be looked up using `info line -function my_coroutine -label
__coro_resume_1`. LLDB unfortunately does not understand DW_TAG_label
debug information, yet.

Given this is an artificial compiler-generated label, I did apply the
DW_AT_artificial tag to it. The DWARFv5 standard only allows that tag on
type and variable definitions, but this is a natural extension and was
also blessed in the RFC on discourse.

Also, this commit adds `DW_AT_decl_column` to labels, not only for
coroutines but also for normal C and C++ labels. While not strictly
necessary, I am doing so now because it would be harder to do so later
without breaking the binary LLVM-IR format

Drive-by fixes: While reading the existing test cases to understand how
to write my own test case, I did a couple of small typo fixes and
comment improvements

1 files changed, 18 insertions, 4 deletions

diff --git a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
index d28ab7d..690341f 100644
--- a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
+++ b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
@@ -2242,14 +2242,28 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     break;
   }
   case bitc::METADATA_LABEL: {
-    if (Record.size() != 5)
+    if (Record.size() < 5 || Record.size() > 7)
       return error("Invalid record");
 
     IsDistinct = Record[0] & 1;
+    uint64_t Line = Record[4];
+    uint64_t Column = Record.size() > 5 ? Record[5] : 0;
+    bool IsArtificial = Record[0] & 2;
+    std::optional<unsigned> CoroSuspendIdx;
+    if (Record.size() > 6) {
+      uint64_t RawSuspendIdx = Record[6];
+      if (RawSuspendIdx != std::numeric_limits<uint64_t>::max()) {
+        if (RawSuspendIdx > (uint64_t)std::numeric_limits<unsigned>::max())
+          return error("CoroSuspendIdx value is too large");
+        CoroSuspendIdx = RawSuspendIdx;
+      }
+    }
+
     MetadataList.assignValue(
-        GET_OR_DISTINCT(DILabel, (Context, getMDOrNull(Record[1]),
-                                  getMDString(Record[2]),
-                                  getMDOrNull(Record[3]), Record[4])),
+        GET_OR_DISTINCT(DILabel,
+                        (Context, getMDOrNull(Record[1]),
+                         getMDString(Record[2]), getMDOrNull(Record[3]), Line,
+                         Column, IsArtificial, CoroSuspendIdx)),
         NextMetadataNo);
     NextMetadataNo++;
     break;