Rollup merge of #147608 - Zalathar:debuginfo, r=nnethercote

cg_llvm: Use `LLVMDIBuilderCreateGlobalVariableExpression`

- Part of rust-lang/rust#134001
- Follow-up to rust-lang/rust#146763

---

This PR dismantles the somewhat complicated `LLVMRustDIBuilderCreateStaticVariable` function, and replaces it with equivalent calls to `LLVMDIBuilderCreateGlobalVariableExpression` and `LLVMGlobalSetMetadata`.

A key difference is that the new code does not replicate the attempted downcast of `InitVal`. As far as I can tell, those downcasts were actually dead, because `llvm::ConstantInt` and `llvm::ConstantFP` are not subclasses of `llvm::GlobalVariable`. I tried replacing those code paths with fatal errors, and was unable to induce failure in any of the relevant test suites I ran.

I have also confirmed that if the calls to `create_static_variable` are commented out, debuginfo tests will fail, demonstrating some amount of relevant test coverage.

The new `DIBuilder` methods have been added via an extension trait, not as inherent methods, to avoid impeding rust-lang/rust#142897.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -22,9 +22,9 @@ use libc::{c_char, c_int, c_uchar, c_uint, c_ulonglong, c_void, size_t};
 
 use super::RustString;
 use super::debuginfo::{
-    DIArray, DIBuilder, DIDerivedType, DIDescriptor, DIEnumerator, DIFile, DIFlags,
-    DIGlobalVariableExpression, DILocation, DISPFlags, DIScope, DISubprogram,
-    DITemplateTypeParameter, DIType, DebugEmissionKind, DebugNameTableKind,
+    DIArray, DIBuilder, DIDerivedType, DIDescriptor, DIEnumerator, DIFile, DIFlags, DILocation,
+    DISPFlags, DIScope, DISubprogram, DITemplateTypeParameter, DIType, DebugEmissionKind,
+    DebugNameTableKind,
 };
 use crate::llvm::MetadataKindId;
 use crate::{TryFromU32, llvm};
@@ -781,7 +781,6 @@ pub(crate) mod debuginfo {
     pub(crate) type DIDerivedType = DIType;
     pub(crate) type DICompositeType = DIDerivedType;
     pub(crate) type DIVariable = DIDescriptor;
-    pub(crate) type DIGlobalVariableExpression = DIDescriptor;
     pub(crate) type DIArray = DIDescriptor;
     pub(crate) type DIEnumerator = DIDescriptor;
     pub(crate) type DITemplateTypeParameter = DIDescriptor;
@@ -1877,6 +1876,22 @@ unsafe extern "C" {
         Length: size_t,
     ) -> &'ll Metadata;
 
+    pub(crate) fn LLVMDIBuilderCreateGlobalVariableExpression<'ll>(
+        Builder: &DIBuilder<'ll>,
+        Scope: Option<&'ll Metadata>,
+        Name: *const c_uchar, // See "PTR_LEN_STR".
+        NameLen: size_t,
+        Linkage: *const c_uchar, // See "PTR_LEN_STR".
+        LinkLen: size_t,
+        File: &'ll Metadata,
+        LineNo: c_uint,
+        Ty: &'ll Metadata,
+        LocalToUnit: llvm::Bool,
+        Expr: &'ll Metadata,
+        Decl: Option<&'ll Metadata>,
+        AlignInBits: u32,
+    ) -> &'ll Metadata;
+
     pub(crate) fn LLVMDIBuilderInsertDeclareRecordAtEnd<'ll>(
         Builder: &DIBuilder<'ll>,
         Storage: &'ll Value,
@@ -2213,22 +2228,6 @@ unsafe extern "C" {
         Ty: &'a DIType,
     ) -> &'a DIType;
 
-    pub(crate) fn LLVMRustDIBuilderCreateStaticVariable<'a>(
-        Builder: &DIBuilder<'a>,
-        Context: Option<&'a DIScope>,
-        Name: *const c_char,
-        NameLen: size_t,
-        LinkageName: *const c_char,
-        LinkageNameLen: size_t,
-        File: &'a DIFile,
-        LineNo: c_uint,
-        Ty: &'a DIType,
-        isLocalToUnit: bool,
-        Val: &'a Value,
-        Decl: Option<&'a DIDescriptor>,
-        AlignInBits: u32,
-    ) -> &'a DIGlobalVariableExpression;
-
     pub(crate) fn LLVMRustDIBuilderCreateEnumerator<'a>(
         Builder: &DIBuilder<'a>,
         Name: *const c_char,