[Support] Remove error return value from one overload of fs::make_absolute

Summary:
The version of make_absolute which accepted a specific directory to use
as the "base" for the computation could never fail, even though it
returned a std::error_code. The reason for that seems to be historical
-- the CWD flavour (which can fail due to failure to retrieve CWD) was
there first, and the new version was implemented by extending that.

This removes the error return value from the non-CWD overload and
reimplements the CWD version on top of that. This enables us to remove
some dead code where people were pessimistically trying to handle the
errors returned from this function.

Reviewers: zturner, sammccall

Subscribers: hiraditya, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D56599

llvm-svn: 351317
diff --git a/llvm/lib/Support/Path.cpp b/llvm/lib/Support/Path.cpp
index a3e6941..5ce2f50 100644
--- a/llvm/lib/Support/Path.cpp
+++ b/llvm/lib/Support/Path.cpp
@@ -849,9 +849,8 @@
   return createTemporaryFile(Prefix, Suffix, Dummy, ResultPath, FS_Name);
 }
 
-static std::error_code make_absolute(const Twine &current_directory,
-                                     SmallVectorImpl<char> &path,
-                                     bool use_current_directory) {
+void make_absolute(const Twine &current_directory,
+                   SmallVectorImpl<char> &path) {
   StringRef p(path.data(), path.size());
 
   bool rootDirectory = path::has_root_directory(p);
@@ -860,14 +859,11 @@
 
   // Already absolute.
   if (rootName && rootDirectory)
-    return std::error_code();
+    return;
 
   // All of the following conditions will need the current directory.
   SmallString<128> current_dir;
-  if (use_current_directory)
-    current_directory.toVector(current_dir);
-  else if (std::error_code ec = current_path(current_dir))
-    return ec;
+  current_directory.toVector(current_dir);
 
   // Relative path. Prepend the current directory.
   if (!rootName && !rootDirectory) {
@@ -875,7 +871,7 @@
     path::append(current_dir, p);
     // Set path to the result.
     path.swap(current_dir);
-    return std::error_code();
+    return;
   }
 
   if (!rootName && rootDirectory) {
@@ -884,7 +880,7 @@
     path::append(curDirRootName, p);
     // Set path to the result.
     path.swap(curDirRootName);
-    return std::error_code();
+    return;
   }
 
   if (rootName && !rootDirectory) {
@@ -896,20 +892,23 @@
     SmallString<128> res;
     path::append(res, pRootName, bRootDirectory, bRelativePath, pRelativePath);
     path.swap(res);
-    return std::error_code();
+    return;
   }
 
   llvm_unreachable("All rootName and rootDirectory combinations should have "
                    "occurred above!");
 }
 
-std::error_code make_absolute(const Twine &current_directory,
-                              SmallVectorImpl<char> &path) {
-  return make_absolute(current_directory, path, true);
-}
-
 std::error_code make_absolute(SmallVectorImpl<char> &path) {
-  return make_absolute(Twine(), path, false);
+  if (path::is_absolute(path))
+    return {};
+
+  SmallString<128> current_dir;
+  if (std::error_code ec = current_path(current_dir))
+    return ec;
+
+  make_absolute(current_dir, path);
+  return {};
 }
 
 std::error_code create_directories(const Twine &Path, bool IgnoreExisting,