Changing include paths used in the public repo.

This CL ensures that `#include`s of `absl` headers use a path similar to
what is used in https://github.com/abseil/abseil-cpp - e.g.: `#include
"absl/strings/ascii.h"`.

Similarily, for LLVM headers, this CL changes their `#include` path to
what is used in https://github.com/llvm/llvm-project - e.g.: `#include
"clang/AST/Decl.h"` or `#include "llvm/Support/ErrorHandling.h"`.

This should help build Crubit in a scenario where its dependencies (i.e.
Abseil and LLVM/Clang) are at different paths than the ones used prior
this CL.

PiperOrigin-RevId: 444899882
diff --git a/common/file_io.h b/common/file_io.h
index 6de3d84..7ab9ff8 100644
--- a/common/file_io.h
+++ b/common/file_io.h
@@ -7,7 +7,7 @@
 
 #include <string>
 
-#include "third_party/absl/status/statusor.h"
+#include "absl/status/statusor.h"
 
 namespace crubit {