[ADT/STLExtras.h] - Add llvm::is_sorted wrapper and update callers.

It can be used to avoid passing the begin and end of a range.
This makes the code shorter and it is consistent with another
wrappers we already have.

Differential revision: https://reviews.llvm.org/D78016
diff --git a/llvm/tools/llvm-exegesis/lib/Target.cpp b/llvm/tools/llvm-exegesis/lib/Target.cpp
index 7fb2231..4e0de93 100644
--- a/llvm/tools/llvm-exegesis/lib/Target.cpp
+++ b/llvm/tools/llvm-exegesis/lib/Target.cpp
@@ -107,8 +107,8 @@
                                                   0u};
 
 const PfmCountersInfo &ExegesisTarget::getPfmCounters(StringRef CpuName) const {
-  assert(std::is_sorted(
-             CpuPfmCounters.begin(), CpuPfmCounters.end(),
+  assert(llvm::is_sorted(
+             CpuPfmCounters,
              [](const CpuAndPfmCounters &LHS, const CpuAndPfmCounters &RHS) {
                return strcmp(LHS.CpuName, RHS.CpuName) < 0;
              }) &&