[MC] Use reportError for .uleb128/.sleb128 diagnostic

User errors should use reportError. reportError allows us to continue parsing
the file and collect more diagnostics.

MC/ELF/leb128-err.s is adapted from MC/RISCV/riscv64-64b-pcrel.s

Author: MaskRay

llvm/lib/MC/MCAssembler.cpp

@@ -1009,8 +1009,11 @@ bool MCAssembler::relaxLEB(MCAsmLayout &Layout, MCLEBFragment &LF) {
   uint64_t OldSize = LF.getContents().size();
   int64_t Value;
   bool Abs = LF.getValue().evaluateKnownAbsolute(Value, Layout);
-  if (!Abs)
-    report_fatal_error("sleb128 and uleb128 expressions must be absolute");
+  if (!Abs) {
+    getContext().reportError(LF.getValue().getLoc(),
+                             Twine(LF.isSigned() ? ".s" : ".u") +
+                                 "leb128 expression is not absolute");
+  }
   SmallString<8> &Data = LF.getContents();
   Data.clear();
   raw_svector_ostream OSE(Data);

llvm/test/MC/ELF/leb128-err.s

@@ -0,0 +1,25 @@
+# RUN: not llvm-mc -filetype=obj -triple=x86_64 %s -o /dev/null 2>&1 | FileCheck %s --implicit-check-not=error:
+
+.section .alloc_w,"aw",@progbits; w:
+# CHECK: :[[#@LINE+1]]:16: error: .uleb128 expression is not absolute
+.uleb128 extern-w   # extern is undefined
+# CHECK: :[[#@LINE+1]]:11: error: .uleb128 expression is not absolute
+.uleb128 w-extern
+# CHECK: :[[#@LINE+1]]:11: error: .uleb128 expression is not absolute
+.uleb128 x-w        # x is later defined in another section
+.uleb128 w1-w       # w1 is later defined in the same section
+w1:
+
+.section .alloc_x,"aw",@progbits; x:
+# CHECK: :[[#@LINE+1]]:11: error: .sleb128 expression is not absolute
+.sleb128 y-x
+.section .alloc_y,"aw",@progbits; y:
+# CHECK: :[[#@LINE+1]]:11: error: .sleb128 expression is not absolute
+.sleb128 x-y
+
+.section .nonalloc_x; nx:
+# CHECK: :[[#@LINE+1]]:12: error: .sleb128 expression is not absolute
+.sleb128 ny-nx
+.section .nonalloc_y; ny:
+# CHECK: :[[#@LINE+1]]:12: error: .sleb128 expression is not absolute
+.sleb128 nx-ny