Update to the latest version of llvm/clang/lld

This contains the following changes relevant for mingw usage:
- A fix in a code generation bug affecting dwarf exception handling
  on i686
- Setting the proper machine type in weak alias objects in import libraries,
  making GNU binutils not reject such files outright
- Fixes for SEH generation for AArch64, for functions containing
  jump tables
- Fixes to the calling convention of functions returning structs on AArch64
- Improvements in handling of dllexport of C++ template instantiations,
  allowing building libc++ into a DLL without using --export-all-symbols
- Proper separators in the output of clang --print-search-dirs on
  Windows
- A new flag --appcontainer to lld's mingw frontend
- Lld now has extra handling of crtend.o, fixing use of lld with gcc
  based mingw setups, when using dwarf exceptions

Author: mstorsjo

build-llvm.sh

@@ -61,13 +61,13 @@ fi
 if [ -n "$SYNC" ] || [ -n "$CHECKOUT" ]; then
     cd llvm
     [ -z "$SYNC" ] || git fetch
-    git checkout 0502ddb2a607516ef3cbd0fb8a67ca483ceaff81
+    git checkout 6bbf836d66c1c71287773dc2201ab1b416050d80
     cd tools/clang
     [ -z "$SYNC" ] || git fetch
-    git checkout 1437396543aad71aa26679ff366cf0266d6a4e0d
+    git checkout 39af588aaa995c092806bbb467680649b858d82b
     cd ../lld
     [ -z "$SYNC" ] || git fetch
-    git checkout 1ea73084c424b2957c3ea7b788916e7206fd597b
+    git checkout fa3678e53a6173fa86edc6c3d60b86d9fa11f0fd
     cd ../../..
 fi