cmake: improve heuristics for static LLVM detection

In 89146e8, a heuristic was introduced where
if libLLVMSupport is available as a static library, LLVM itself is assumed
to be availble as a static library as a whole.

This is unfortunately not the case at least on Gentoo and Arch Linux, where
a subsequent llvm-config call yields:
```
$ /usr/lib/llvm/17/bin/llvm-config --libfiles --link-static
llvm-config: error: missing: /usr/lib/llvm/17/lib64/libLLVMTargetParser.a
llvm-config: error: missing: /usr/lib/llvm/17/lib64/libLLVMBinaryFormat.a
llvm-config: error: missing: /usr/lib/llvm/17/lib64/libLLVMBitstreamReader.a
llvm-config: error: missing: /usr/lib/llvm/17/lib64/libLLVMRemarks.a
[...]
```

On Gentoo, where LLVM's static libraries are not included, we still have:
```
$ equery f llvm:17 | grep -i lib64/.*.a$
/usr/lib/llvm/17/lib64/libLLVMDemangle.a
/usr/lib/llvm/17/lib64/libLLVMSupport.a
/usr/lib/llvm/17/lib64/libLLVMTableGen.a
/usr/lib/llvm/17/lib64/libLLVMTestingAnnotations.a
/usr/lib/llvm/17/lib64/libLLVMTestingSupport.a
/usr/lib/llvm/17/lib64/libllvm_gtest.a
/usr/lib/llvm/17/lib64/libllvm_gtest_main.a
```

Therefore, testing for libLLVMSupport is insufficient. We now try libLLVMCore
instead, as that appears to only be installed when LLVM truly has static libraries
available. libLLVMCore is handled as a LLVM component which gives us some guarantee
this is supposed to be happening and not a fluke.

(Specifically, LLVM's llvm/lib/Support/CMakeLists.txt pays 0 attention to
-DLLVM_BUILD_LLVM_DYLIB and -DLLVM_LINK_LLVM_DYLIB, and is hence only affected
by -DBUILD_SHARED_LIBS, which LLVM upstream say is only to be used for development.

Therefore, with -DBUILD_SHARED_LIBS=OFF (as is recommended/the default) and
-DLLVM_BUILD_LLVM_DYLIB=ON, you will get a static libLLVMSupport, without it
indicating anything about the rest of your configuration.)

Closes: mesonbuild#12323
Fixes: 89146e8
Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Eli Schwartz <eschwartz93@gmail.com>

mesonbuild/dependencies/data/CMakeListsLLVM.txt

@@ -85,7 +85,7 @@ if(LLVM_FOUND)
   elseif(NOT LLVM_MESON_DYLIB)
     # Use LLVMSupport to check if static targets exist
     set(static_tg FALSE)
-    is_static(LLVMSupport static_tg)
+    is_static(LLVMCore static_tg)
     if(static_tg)
       set(PACKAGE_FOUND TRUE)
     endif(static_tg)