Correctly mark parameter RustMappingRegions as pointer-to-const

The regions don't need to be mutable because we pass a copy of them to LLVM
instead, and this matches the `*const` in the Rust-side signature.

Author: Zalathar

compiler/rustc_llvm/llvm-wrapper/CoverageMappingWrapper.cpp

@@ -121,7 +121,7 @@ extern "C" void LLVMRustCoverageWriteMappingToBuffer(
     unsigned NumVirtualFileMappingIDs,
     const LLVMRustCounterExpression *RustExpressions,
     unsigned NumExpressions,
-    LLVMRustCounterMappingRegion *RustMappingRegions,
+    const LLVMRustCounterMappingRegion *RustMappingRegions,
     unsigned NumMappingRegions,
     RustStringRef BufferOut) {
   // Convert from FFI representation to LLVM representation.