[lld][WebAssembly] Give better warnings on bad relocation sites

[sbc100]

Summary

This should help when folks report bugs about this warning/error:
https://github.com/emscripten-core/emscripten/issues/8731

lld/test/wasm/bad-reloc.yaml

+# RUN: yaml2obj %s -o %t.o
+# RUN: not wasm-ld -o out.wasm %t.o 2>&1 | FileCheck %s
+#
+# CHECK: bad-reloc.yaml.tmp.o:(_start): unexpected existing value for R_WASM_MEMORY_ADDR_LEB: existing=1 expected=0
+#
+# The code section bellow has been modified such that the target of the
+# relocation is 8180808000 (LEB of 1) rather than 8080808000 (LEB = 0)
+
+--- !WASM
+FileHeader:
+  Version:         0x00000001
+Sections:
+  - Type:            TYPE
+    Signatures:
+      - Index:           0
+        ParamTypes:      []
+        ReturnTypes:
+          - I32
+  - Type:            IMPORT
+    Imports:
+      - Module:          env
+        Field:           __linear_memory
+        Kind:            MEMORY
+        Memory:
+          Initial:         0x00000001
+      - Module:          env
+        Field:           __indirect_function_table
+        Kind:            TABLE
+        Table:
+          Index:           0
+          ElemType:        FUNCREF
+          Limits:
+            Initial:         0x00000000
+  - Type:            FUNCTION
+    FunctionTypes:   [ 0 ]
+  - Type:            DATACOUNT
+    Count:           1
+  - Type:            CODE
+    Relocations:
+      - Type:            R_WASM_MEMORY_ADDR_LEB
+        Index:           1
+        Offset:          0x00000007
+    Functions:
+      - Index:           0
+        Locals:          []
+        Body:            4100280281808080002802000B
+  - Type:            DATA
+    Segments:
+      - SectionOffset:   6
+        InitFlags:       0
+        Offset:
+          Opcode:          I32_CONST
+          Value:           0
+        Content:         '00000000'
+  - Type:            CUSTOM
+    Name:            linking
+    Version:         2
+    SymbolTable:
+      - Index:           0
+        Kind:            FUNCTION
+        Name:            _start
+        Flags:           [ VISIBILITY_HIDDEN ]
+        Function:        0
+      - Index:           1
+        Kind:            DATA
+        Name:            bar
+        Flags:           [ VISIBILITY_HIDDEN ]
+        Segment:         0
+        Size:            4
+    SegmentInfo:
+      - Index:           0
+        Name:            .bss.bar
+        Alignment:       2
+        Flags:           [  ]
+

lld/wasm/InputChunks.cpp

for (const WasmRelocation &rel : relocations) {
 
     if (bytesRead && bytesRead != 5)
       warn("expected LEB at relocation site be 5-byte padded");
 
     if (rel.Type != R_WASM_GLOBAL_INDEX_LEB &&
         rel.Type != R_WASM_GLOBAL_INDEX_I32) {
       auto expectedValue = file->calcExpectedValue(rel);
       if (expectedValue != existingValue)
-        warn("unexpected existing value for " + relocTypeToString(rel.Type) +
-             ": existing=" + Twine(existingValue) +
-             " expected=" + Twine(expectedValue));
+        warn(toString(this) + ": unexpected existing value for " +
+             relocTypeToString(rel.Type) + ": existing=" +
+             Twine(existingValue) + " expected=" + Twine(expectedValue));
     }
   }
 }
 
 // Copy this input chunk to an mmap'ed output file and apply relocations.
 void InputChunk::writeTo(uint8_t *buf) const {
   // Copy contents
   memcpy(buf + outputOffset, data().data(), data().size());