[NFC]Codestyle changes for SampleFDO library (#147840)

* Introduce an error code for illegal_line_offset in sampleprof_error
namespace, and use it for line offset parsing error.
* Add `const` for `LineLocation::serialize`.
* Use structured binding, make_first/second_range in loops.

I'm working on a [sample-profile format
change](https://github.com/llvm/llvm-project/compare/users/mingmingl-llvm/samplefdo-profile-format)
to extend SampleFDO profile with vtable profiles. And this change splits
the non-functional changes.

1 files changed, 2 insertions, 2 deletions

diff --git a/llvm/lib/ProfileData/SampleProfReader.cpp b/llvm/lib/ProfileData/SampleProfReader.cpp
index 6466618..cf78740 100644
--- a/llvm/lib/ProfileData/SampleProfReader.cpp
+++ b/llvm/lib/ProfileData/SampleProfReader.cpp
@@ -609,7 +609,7 @@ SampleProfileReaderBinary::readProfile(FunctionSamples &FProfile) {
       return EC;
 
     if (!isOffsetLegal(*LineOffset)) {
-      return std::error_code();
+      return sampleprof_error::illegal_line_offset;
     }
 
     auto Discriminator = readNumber<uint64_t>();
@@ -1236,7 +1236,7 @@ std::error_code SampleProfileReaderExtBinaryBase::readCSNameTableSec() {
         return EC;
 
       if (!isOffsetLegal(*LineOffset))
-        return std::error_code();
+        return sampleprof_error::illegal_line_offset;
 
       auto Discriminator = readNumber<uint64_t>();
       if (std::error_code EC = Discriminator.getError())