Change the PathMappingList::FindFile to use FileSpec API's

Also, when appending path components, collapse multiple "/" into one at the join.

llvm-svn: 279533
diff --git a/lldb/source/Host/common/FileSpec.cpp b/lldb/source/Host/common/FileSpec.cpp
index 53c0ab4..62117b78 100644
--- a/lldb/source/Host/common/FileSpec.cpp
+++ b/lldb/source/Host/common/FileSpec.cpp
@@ -1552,6 +1552,9 @@
             stream.PutChar(GetPrefferedPathSeparator(m_syntax));
     }
 
+    while (IsPathSeparator(new_path[0], m_syntax))
+        new_path++;
+
     stream.PutCString(new_path);
 
     const bool resolve = false;