[ELF] -r: allow SHT_X86_64_UNWIND to be merged into SHT_PROGBITS

* For .cfi_*, GCC/GNU as emits SHT_PROGBITS type .eh_frame sections.
* Since rL252300, clang emits SHT_X86_64_UNWIND type .eh_frame sections
  (originated from Solaris, documented in the x86-64 psABI).
* Some assembly use `.section .eh_frame,"a",@unwind` to generate
  SHT_X86_64_UNWIND .eh_frame sections.

In a non-relocatable link, input .eh_frame are combined and there is
only one SyntheticSection .eh_frame in the output section, so the
"section type mismatch" diagnostic does not fire.

In a relocatable link, there is no SyntheticSection .eh_frame. .eh_frame of
mixed types can trigger the diagnostic. This patch fixes it by adding another
special case 0x70000001 (= SHT_X86_64_UNWIND) to canMergeToProgbits().

    ld.lld -r gcc.o clang.o => error: section type mismatch for .eh_frame

There was a discussion "RFC: Usefulness of SHT_X86_64_UNWIND" on the x86-64-abi
mailing list. Folks are not wild about making the psABI value 0x70000001 into
gABI, but a few think defining 0x70000001 for .eh_frame may be a good idea for a
new architecture.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D85785

lld/ELF/OutputSections.cpp

@@ -78,10 +78,14 @@ OutputSection::OutputSection(StringRef name, uint32_t type, uint64_t flags)
 // to be allocated for nobits sections. Other ones don't require
 // any special treatment on top of progbits, so there doesn't
 // seem to be a harm in merging them.
+//
+// NOTE: clang since rL252300 emits SHT_X86_64_UNWIND .eh_frame sections. Allow
+// them to be merged into SHT_PROGBITS .eh_frame (GNU as .cfi_*).
 static bool canMergeToProgbits(unsigned type) {
   return type == SHT_NOBITS || type == SHT_PROGBITS || type == SHT_INIT_ARRAY ||
          type == SHT_PREINIT_ARRAY || type == SHT_FINI_ARRAY ||
-         type == SHT_NOTE;
+         type == SHT_NOTE ||
+         (type == SHT_X86_64_UNWIND && config->emachine == EM_X86_64);
 }
 
 // Record that isec will be placed in the OutputSection. isec does not become

lld/test/ELF/eh-frame-type.test

@@ -11,6 +11,9 @@
 # RUN: ld.lld %t1.o %t2.o -o %tboth
 # RUN: llvm-readobj -S %tboth | FileCheck %s
 
+# RUN: ld.lld -r %t1.o %t2.o -o %tboth.ro
+# RUN: llvm-readobj -S %tboth.ro | FileCheck %s
+
 # CHECK:      Name: .eh_frame
 # CHECK-NEXT: Type: SHT_PROGBITS