IRToASTVisitor: preserve external LLVM global declarations as C extern #352

Rellic emits @counter = external global i32 as unsigned int counter;.
That C declaration creates a tentative definition. Linking the emitted C
with the original data provider therefore fails with a duplicate symbol.

target triple = "i686-pc-windows-msvc"
@counter = external global i32
define i32 @read_counter() {
  %value = load i32, ptr @counter
  ret i32 %value
}

Expected output is extern unsigned int counter;. The narrow fix passes
SC_Extern to CreateVarDecl when the LLVM global is a declaration with
ordinary external linkage. Initialized definitions keep their existing
storage class and initializer.

The independently synthetic fixture in tests/external-global-declarations/
contains five complete functions, three external globals and one owned global
initialized to 11. A separate provider defines the external data. With
Clang/LLVM 17.0.6, Windows i686 and -fno-common, the original LLVM module
links with the provider; baseline emitted C fails with three duplicate
definitions. Corrected emitted C links, retains three undefined consumer
symbols and preserves the owned initializer. Linking without the provider
fails. Reccmp compares all five function bodies; all are byte-identical.
No runtime execution or argument sampling is performed.

Standalone whole modules: https://github.com/humdrum00001010/rellic/tree/dd9d094f7bad8c23c87833e7dd8171c344ee83d0/tests/external-global-declarations

Generic fix: https://github.com/humdrum00001010/rellic/commit/dd9d094f7bad8c23c87833e7dd8171c344ee83d0