[llvm-rc] Use proper search algorithm for finding resources.

Previously we would only look in the current directory for a
resource, which might not be the same as the directory of the
rc file.  Furthermore, MSVC rc supports a /I option, and can
also look in the system environment.  This patch adds support
for this search algorithm.

Differential Revision: https://reviews.llvm.org/D38740

llvm-svn: 315499
diff --git a/llvm/tools/llvm-rc/ResourceScriptParser.h b/llvm/tools/llvm-rc/ResourceScriptParser.h
index 1a124d4..84fdfd5 100644
--- a/llvm/tools/llvm-rc/ResourceScriptParser.h
+++ b/llvm/tools/llvm-rc/ResourceScriptParser.h
@@ -25,6 +25,9 @@
 #include <vector>
 
 namespace llvm {
+namespace opt {
+class InputArgList;
+}
 namespace rc {
 
 class RCParser {
@@ -51,7 +54,7 @@
     LocIter ErrorLoc, FileEnd;
   };
 
-  RCParser(std::vector<RCToken> TokenList);
+  explicit RCParser(std::vector<RCToken> TokenList);
 
   // Reads and returns a single resource definition, or error message if any
   // occurred.