build-llvm: Update to the 10.0.0 release of llvm

Notable changes since the previously pinned (early December 2019)
version:
- Implicitly add the .exe suffix if not provided in the clang
  -o argument, when running on windows (to match GCC's behaviour)
- Fixes for accesses to TLS variables in another object file
- Fixes for accessing weak functions that may be undefined, on ARM and AArch64
  (x86 was fixed earlier)
- Fixed many instances of locale handling in libcxx

build-llvm.sh

@@ -16,7 +16,7 @@
 
 set -e
 
-: ${LLVM_VERSION:=c49194969430f0ee817498a7000a979a7a0ded03}
+: ${LLVM_VERSION:=llvmorg-10.0.0}
 ASSERTS=OFF
 BUILDDIR=build
 unset HOST