build-llvm: Update to the 11.0.0 release of llvm

Notable changes since LLVM 10.0.0:

- Enable -mms-bitfields by default (like GCC), don't produce errors
  if the -mms-bitfields option is used together with non-trivial C++
  classes
- Support for more linker flags in LLD (--disable-auto-import,
  --disable-runtime-pseudo-reloc, --file-alignment, --section-alignment),
  and fixed the handling of the --no-seh flag
- Clearly error out on unsupported relocation types in aarch64 assembly
- Proper support for __attribute__((weak))
- Minor improvements to details in llvm-rc
- Fixed a regression in LLD about exporting symbols containing periods ('.')
  with a def file

build-llvm.sh

@@ -16,7 +16,7 @@
 
 set -e
 
-: ${LLVM_VERSION:=llvmorg-10.0.0}
+: ${LLVM_VERSION:=llvmorg-11.0.0}
 ASSERTS=OFF
 BUILDDIR=build
 unset HOST