[ItaniumDemangle][test] Factor demangler test-cases into file and sync into LLVM (#137947)

This patch turns the `libcxxabi/test/test_demangle.pass.cpp` into gtest
unit-tests in `llvm/unittests/Demangle`. The main motivation for this is
#137793, where we want to
re-use the test-cases from the ItaniumDemangler to test our OutputBuffer
implementation.

`libcxxabi/test/test_demangle.pass.cpp` now only tests the
`__cxa_demangle` API surface, not the underlying ItaniumDemangle
implementation.

Author: Michael137

libcxxabi/src/demangle/cp-to-llvm.sh

@@ -7,30 +7,52 @@ set -e
 
 cd $(dirname $0)
 HDRS="ItaniumDemangle.h ItaniumNodes.def StringViewExtras.h Utility.h"
+TEST_HDRS="DemangleTestCases.inc"
 LLVM_DEMANGLE_DIR=$1
+LLVM_TESTING_DIR=
 
 if [[ -z "$LLVM_DEMANGLE_DIR" ]]; then
     LLVM_DEMANGLE_DIR="../../../llvm/include/llvm/Demangle"
+    LLVM_TESTING_DIR=$LLVM_DEMANGLE_DIR/../Testing/Demangle
 fi
 
 if [[ ! -d "$LLVM_DEMANGLE_DIR" ]]; then
     echo "No such directory: $LLVM_DEMANGLE_DIR" >&2
     exit 1
 fi
 
-read -p "This will overwrite the copies of $HDRS in $LLVM_DEMANGLE_DIR; are you sure? [y/N]" -n 1 -r ANSWER
+if [[ ! -d "$LLVM_TESTING_DIR" ]]; then
+    LLVM_TESTING_DIR="../../../llvm/include/llvm/Testing/Demangle"
+fi
+
+if [[ ! -d "$LLVM_TESTING_DIR" ]]; then
+    echo "No such directory: $LLVM_TESTING_DIR" >&2
+    exit 1
+fi
+
+read -p "This will overwrite the copies of $HDRS in $LLVM_DEMANGLE_DIR and $TEST_HDRS in $LLVM_TESTING_DIR; are you sure? [y/N]" -n 1 -r ANSWER
 echo
 
-if [[ $ANSWER =~ ^[Yy]$ ]]; then
-    cp -f README.txt $LLVM_DEMANGLE_DIR
-    chmod -w $LLVM_DEMANGLE_DIR/README.txt
-    for I in $HDRS ; do
-	rm -f $LLVM_DEMANGLE_DIR/$I
-	dash=$(echo "$I---------------------------" | cut -c -27 |\
-		   sed 's|[^-]*||')
-	sed -e '1s|^//=*-* .*\..* -*.*=*// *$|//===--- '"$I $dash"'-*- mode:c++;eval:(read-only-mode) -*-===//|' \
-	    -e '2s|^// *$|//       Do not edit! See README.txt.|' \
-	    $I >$LLVM_DEMANGLE_DIR/$I
-	chmod -w $LLVM_DEMANGLE_DIR/$I
+copy_files() {
+    local src=$1
+    local dst=$2
+    local hdrs=$3
+
+    cp -f README.txt $dst
+    chmod -w $dst/README.txt
+
+    for I in $hdrs ; do
+	    rm -f $dst/$I
+	    dash=$(echo "$I---------------------------" | cut -c -27 |\
+		       sed 's|[^-]*||')
+	    sed -e '1s|^//=*-* .*\..* -*.*=*// *$|//===--- '"$I $dash"'-*- mode:c++;eval:(read-only-mode) -*-===//|' \
+	        -e '2s|^// *$|//       Do not edit! See README.txt.|' \
+	        $src/$I >$dst/$I
+	    chmod -w $dst/$I
     done
+}
+
+if [[ $ANSWER =~ ^[Yy]$ ]]; then
+    copy_files . $LLVM_DEMANGLE_DIR $HDRS
+    copy_files ../../test $LLVM_TESTING_DIR $TEST_HDRS
 fi