[llvm-ar] Make paths case insensitive when on windows

When on windows gnu-ar treats member names as case insensitive. This
commit implements the same behaviour.

Differential Revision: https://reviews.llvm.org/D68033

llvm-svn: 375002
diff --git a/llvm/docs/CommandGuide/llvm-ar.rst b/llvm/docs/CommandGuide/llvm-ar.rst
index 8067f7e..1073d96 100644
--- a/llvm/docs/CommandGuide/llvm-ar.rst
+++ b/llvm/docs/CommandGuide/llvm-ar.rst
@@ -35,6 +35,12 @@
 
  Currently **llvm-ar** can read GNU and BSD long file names, but only writes
  archives with the GNU format.
+ 
+*Windows Paths*
+
+ When on Windows **llvm-ar** treats the names of archived *files* in the same
+ case sensitive manner as the operating system. When on a non-Windows machine
+ **llvm-ar** does not consider character case.
 
 OPTIONS
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