[MC] Properly report errors for .subsection

For the out-of-range error, MCConstantExpr doesn't have a location, so we
can only show "<unknown>:0:".

Also, allow subsection numbers up to 2147483647, which is the maximum value GNU
assembler supports. (GNU assembler also supports negative numbers.)

llvm/lib/MC/MCObjectStreamer.cpp

@@ -376,10 +376,15 @@ bool MCObjectStreamer::changeSectionImpl(MCSection *Section,
 
   int64_t IntSubsection = 0;
   if (Subsection &&
-      !Subsection->evaluateAsAbsolute(IntSubsection, getAssemblerPtr()))
-    report_fatal_error("Cannot evaluate subsection number");
-  if (IntSubsection < 0 || IntSubsection > 8192)
-    report_fatal_error("Subsection number out of range");
+      !Subsection->evaluateAsAbsolute(IntSubsection, getAssemblerPtr())) {
+    getContext().reportError(Subsection->getLoc(),
+                             "cannot evaluate subsection number");
+  }
+  if (!isUInt<31>(IntSubsection)) {
+    getContext().reportError(Subsection->getLoc(),
+                             "subsection number must be within [0,2147483647]");
+  }
+
   CurSubsectionIdx = unsigned(IntSubsection);
   CurInsertionPoint =
       Section->getSubsectionInsertionPoint(CurSubsectionIdx);

llvm/test/MC/ELF/subsection.s

@@ -1,4 +1,5 @@
 // RUN: llvm-mc -filetype=obj %s -o - -triple x86_64-pc-linux | llvm-objdump -s - | FileCheck %s
+// RUN: not llvm-mc -filetype=obj -triple x86_64 --defsym ERR=1 %s -o /dev/null 2>&1 | FileCheck %s --check-prefix=ERR --implicit-check-not=error:
 
 // CHECK: Contents of section .text:
 // CHECK-NEXT: 0000 03042502 00000003 04250100 0000ebf7
@@ -35,3 +36,15 @@ l2:
 .byte 2
 .popsection
 .byte 3
+
+
+.ifdef ERR
+// ERR: :[[#@LINE+1]]:13: error: cannot evaluate subsection number
+.subsection l2
+
+// ERR: <unknown>:0: error: subsection number must be within [0,2147483647]
+.subsection 0-1
+.subsection 2147483647
+// ERR: <unknown>:0: error: subsection number must be within [0,2147483647]
+.subsection 2147483648
+.endif