Added explicit use of lld and llvm-ar to msys2 github workflow

The clangx64ninja build test env explicitly specified only the clang/++
compiler env.vars (CC, CXX, OBJC, OBJCXX), which leaves meson to guess
at what static & dynamic linkers to use.  Unfortunately, it can decide
to go for the gnu 'ld.bfd' linker, which might sometimes be compatible
with clang-compiled objects but is not compatible with clang's LTO byte-
code output. Now we explictly use all llvm-based tools by adding -
- ..._LD=ld.lld
- AR=llvm-ar

Tried and failed using `..._LD=ld.lld`.  Meson seems to use the _LD
linker env.var value to directly invoke -
`clang -Wl,--version -fuse-ld=ld.lld`
which doesn't work; needs to be 'lld' (`..._LD=lld`) it seems.

.github/workflows/msys2.yml

@@ -83,6 +83,11 @@ jobs:
             mingw-w64-${{ matrix.MSYS2_ARCH }}-python-jsonschema
             mingw-w64-${{ matrix.MSYS2_ARCH }}-${{ matrix.TOOLCHAIN }}
 
+      - name: Conditional lld install
+        run: |
+          pacman -S --noconfirm mingw-w64-${{ matrix.MSYS2_ARCH }}-lld
+        if: ${{ matrix.COMPILER == 'clang' }}
+
       - name: Install dependencies
         run: |
           python3 -m pip --disable-pip-version-check install gcovr pefile pytest pytest-subtests pytest-xdist coverage
@@ -111,9 +116,14 @@ jobs:
 
           if [[ '${{ matrix.COMPILER }}' == 'clang' ]]; then
             export CC=clang
+            export CC_LD=lld
             export CXX=clang++
+            export CXX_LD=lld
             export OBJC=clang
+            export OBJC_LD=lld
             export OBJCXX=clang++
+            export OBJCXX_LD=lld
+            export AR=llvm-ar
           fi
 
           if [[ "${{ matrix.MSYS2_CURSES }}" != "" ]]; then