[Clang] Give error message for invalid profile path when compiling IR

Before this patch, when compiling an IR file (eg the .llvmbc section
from an object file compiled with -Xclang -fembed-bitcode=all) and
profile data was passed in using the -fprofile-instrument-use-path
flag, there would be no error printed (as the previous implementation
relied on the error getting caught again in the constructor of
CodeGenModule which isn't called when -x ir is set). This patch
moves the error checking directly to where the error is caught
originally rather than failing silently in setPGOUseInstrumentor and
waiting to catch it in CodeGenModule to print diagnostic information to
the user.

Regression test added.

Reviewed By: xur, mtrofin

Differential Revision: https://reviews.llvm.org/D132991

clang/lib/CodeGen/CodeGenModule.cpp

@@ -182,15 +182,11 @@ CodeGenModule::CodeGenModule(ASTContext &C,
   if (CodeGenOpts.hasProfileClangUse()) {
     auto ReaderOrErr = llvm::IndexedInstrProfReader::create(
         CodeGenOpts.ProfileInstrumentUsePath, CodeGenOpts.ProfileRemappingFile);
-    if (auto E = ReaderOrErr.takeError()) {
-      unsigned DiagID = Diags.getCustomDiagID(DiagnosticsEngine::Error,
-                                              "Could not read profile %0: %1");
-      llvm::handleAllErrors(std::move(E), [&](const llvm::ErrorInfoBase &EI) {
-        getDiags().Report(DiagID) << CodeGenOpts.ProfileInstrumentUsePath
-                                  << EI.message();
-      });
-    } else
-      PGOReader = std::move(ReaderOrErr.get());
+    // We're checking for profile read errors in CompilerInvocation, so if
+    // there was an error it should've already been caught. If it hasn't been
+    // somehow, trip an assertion.
+    assert(ReaderOrErr);
+    PGOReader = std::move(ReaderOrErr.get());
   }
 
   // If coverage mapping generation is enabled, create the