[llvm-cov] Add error for invalid -path-equivalence format

[keith]

Only question for folks is if mapping foo to an empty string did anything intentional?

[keith]

Mind taking a look?

[phosek]

LGTM

llvm/test/tools/llvm-cov/path_equivalence.c

 // RUN: llvm-profdata merge %S/Inputs/path_equivalence.proftext -o %t.profdata
 // RUN: llvm-cov show %S/Inputs/path_equivalence.covmapping -instr-profile=%t.profdata -path-equivalence=/tmp,%S | FileCheck %s
 // RUN: llvm-cov show %S/Inputs/path_equivalence.covmapping -instr-profile=%t.profdata -path-equivalence=/tmp/,%S/ | FileCheck %s
 int main() {} // CHECK: [[@LINE]]|      1|int main() {}
+
+// RUN: not llvm-cov show --instr-profile=/dev/null -path-equivalence=foo /dev/null 2>&1 | FileCheck --check-prefix=INVALID %s
+// INVALID: error: -path-equivalence: invalid argument 'foo', must be in format 'from,to'

llvm/tools/llvm-cov/CodeCoverage.cpp

case CoverageViewOptions::OutputFormat::Lcov:
       if (UseColor == cl::BOU_TRUE)
         errs() << "Color output cannot be enabled when generating lcov.\n";
       ViewOpts.Colors = false;
       break;
     }
 
     // If path-equivalence was given and is a comma seperated pair then set
     // PathRemapping.
+    if (!PathRemap.empty()) {
       auto EquivPair = StringRef(PathRemap).split(',');
-    if (!(EquivPair.first.empty() && EquivPair.second.empty()))
+      if (EquivPair.first.empty() || EquivPair.second.empty()) {
+        error("invalid argument '" + PathRemap +
+                  "', must be in format 'from,to'",
+              "-path-equivalence");
+        return 1;
+      }
+
       PathRemapping = {std::string(EquivPair.first),
                        std::string(EquivPair.second)};
+    }
 
     // If a demangler is supplied, check if it exists and register it.
     if (!DemanglerOpts.empty()) {
       auto DemanglerPathOrErr = sys::findProgramByName(DemanglerOpts[0]);
       if (!DemanglerPathOrErr) {
         error("Could not find the demangler!",
               DemanglerPathOrErr.getError().message());
         return 1;