[llvm-cov][WebAssembly] Align coverage mapping section to 8 bytes

Wasm format does not have a good way to insert padding bytes to align
the start of a section contents because the size of each section header
depends on the size of the section contents (leb128 encoding). This
design makes it difficult to align section contents to a specific
alignment to load them by post-processing tools like llvm-cov.
This patch copies the coverage mapping section to a new buffer that is
aligned to 8 bytes if the original section is not aligned. This is
not an ideal solution but it works for now.

Author: kateinoigakukun

llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp

@@ -1136,6 +1136,16 @@ loadBinaryFormat(std::unique_ptr<Binary> Bin, StringRef Arch,
     return CoverageMappingOrErr.takeError();
   StringRef CoverageMapping = CoverageMappingOrErr.get();
 
+  // If the coverage mapping section is not aligned to 8 bytes, copy it to a
+  // new buffer that is. Wasm format typically has unaligned section contents
+  // because it doesn't have a good way to insert padding bytes.
+  std::unique_ptr<MemoryBuffer> CoverageMappingBufferOwner;
+  if (!isAddrAligned(Align(8), CoverageMapping.data())) {
+    CoverageMappingBufferOwner =
+        MemoryBuffer::getMemBufferCopy(CoverageMapping);
+    CoverageMapping = CoverageMappingBufferOwner->getBuffer();
+  }
+
   // Look for the coverage records section (Version4 only).
   auto CoverageRecordsSections = lookupSections(*OF, IPSK_covfun);