build-llvm: Bump to the final 15.0.0 release

Notable changes are:
- Require strict matches for defines for precompiled headers in GCC
  style PCH directories. This fixes some cases of building DLLs and
  static libraries at the same time with libtool, with PCH.
- Support for the new embedded -exclude-symbols: directive in LLD
- Fix LLD crashes if including object files containing export directives
  via the --undefined/-u option

build-llvm.sh

@@ -16,7 +16,7 @@
 
 set -e
 
-: ${LLVM_VERSION:=llvmorg-15.0.0-rc1}
+: ${LLVM_VERSION:=llvmorg-15.0.0}
 ASSERTS=OFF
 unset HOST
 BUILDDIR="build"