Absolutize paths after remapping (#91)

The index store library requires paths being written are absolute. If we
don't absolutize them then it does. Unfortunately its logic for that
computes the PWD for every single path, which can amount to ~5% of the
total time spent in index-import.

Signed-off-by: Keith Smiley <keithbsmiley@gmail.com>

index-import.cpp

@@ -59,18 +59,33 @@ static cl::opt<bool> UndoRulesSwiftRenames(
 
 struct Remapper {
 public:
+  explicit Remapper() { llvm::sys::fs::current_path(this->pwd); }
+
   std::string remap(const llvm::StringRef input) const {
+    if (input.empty()) {
+      return input.str();
+    }
+
     std::string input_str = input.str();
+    std::optional<StringRef> remapped_str = std::nullopt;
     for (const auto &remap : this->_remaps) {
       const auto &pattern = remap.first;
       const auto &replacement = remap.second;
       if (re2::RE2::Replace(&input_str, *pattern, replacement)) {
-        return path::remove_leading_dotslash(StringRef(input_str)).str();
+        remapped_str = path::remove_leading_dotslash(StringRef(input_str));
+        break;
       }
     }
 
-    // No patterns matched, return the input unaltered.
-    return path::remove_leading_dotslash(input).str();
+    StringRef output_str =
+        remapped_str.value_or(path::remove_leading_dotslash(input));
+    if (path::is_absolute(output_str)) {
+      return output_str.str();
+    }
+
+    SmallString<128> absolute(output_str);
+    llvm::sys::fs::make_absolute(this->pwd, absolute);
+    return absolute.str().str();
   }
 
   void addRemap(std::shared_ptr<re2::RE2> &pattern,
@@ -79,6 +94,9 @@ struct Remapper {
   }
 
   std::vector<std::pair<std::shared_ptr<re2::RE2>, std::string>> _remaps;
+
+private:
+  SmallString<128> pwd;
 };
 
 // Helper for working with index::writer::OpaqueModule. Provides the following: