[FileCheck] Try to fix buildbot failures caused by c7c542e

For example,

<https://lab.llvm.org/buildbot/#/builders/132/builds/3929>

has this diagnostic:

```
/opt/gcc/9.3.0/snos/include/g++/bits/stl_tree.h:780:8: error: static assertion failed: comparison object must be invocable as const
  780 |        is_invocable_v<const _Compare&, const _Key&, const _Key&>,
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

llvm/utils/FileCheck/FileCheck.cpp

@@ -380,7 +380,7 @@ BuildInputAnnotations(const SourceMgr &SM, unsigned CheckFileBufferID,
                       std::vector<InputAnnotation> &Annotations,
                       unsigned &LabelWidth) {
   struct CompareSMLoc {
-    bool operator()(const SMLoc &LHS, const SMLoc &RHS) {
+    bool operator()(const SMLoc &LHS, const SMLoc &RHS) const {
       return LHS.getPointer() < RHS.getPointer();
     }
   };