[BOLT][test] Fix uses of clang C exe for C++ files

The C frontend executable will fail if there's for example a Clang resource file for the C++ mode that sets C++-specific options:
```
+ /home/tambre/dev/llvm/build/bin/clang --target=x86_64-unknown-linux-gnu -fPIE -fuse-ld=lld -Wl,--unresolved-symbols=ignore-all -pie -fPIC -shared /home/tambre/dev/llvm/bolt/test/R_ABS.pic.lld.cpp -o /home/tambre/dev/llvm/build/tools/bolt/test/Output/R_ABS.pic.lld.cpp.tmp.so -Wl,-q -fuse-ld=lld
clang: warning: argument unused during compilation: '-pie' [-Wunused-command-line-argument]
error: invalid argument '-std=c23' not allowed with 'C++'
```

Author: tambry

bolt/test/AArch64/double_jump.cpp

@@ -1,6 +1,6 @@
 // A contrived example to test the double jump removal peephole.
 
-// RUN: %clang %cflags -O0 %s -o %t.exe
+// RUN: %clangxx %cxxflags -O0 %s -o %t.exe
 // RUN: llvm-bolt %t.exe -o %t.bolt --peepholes=double-jumps | \
 // RUN:   FileCheck %s -check-prefix=CHECKBOLT
 // RUN: llvm-objdump --no-print-imm-hex -d %t.bolt | FileCheck %s

bolt/test/R_ABS.pic.lld.cpp

@@ -6,7 +6,7 @@
  * with libc available.
  * REQUIRES: system-linux
  *
- * RUN: %clang %cflags -fPIC -shared %s -o %t.so -Wl,-q -fuse-ld=lld
+ * RUN: %clangxx %cxxflags -fPIC -shared %s -o %t.so -Wl,-q -fuse-ld=lld
  * RUN: llvm-bolt %t.so -o %t.so.bolt --relocs
  */
 

bolt/test/X86/double-jump.test

@@ -4,7 +4,7 @@
 ## correctly on Windows e.g. subshell execution
 REQUIRES: shell
 
-RUN: %clang %cflags %p/Inputs/double_jump.cpp -o %t.exe
+RUN: %clangxx %cxxflags %p/Inputs/double_jump.cpp -o %t.exe
 RUN: (llvm-bolt %t.exe --peepholes=double-jumps \
 RUN:   --eliminate-unreachable -o %t 2>&1 \
 RUN:   && llvm-objdump -d %t --print-imm-hex --no-show-raw-insn) | FileCheck %s

bolt/test/X86/jmp-optimization.test

@@ -4,7 +4,7 @@
 ## correctly on Windows e.g. unsupported parameter expansion
 REQUIRES: shell
 
-RUN: %clang %cflags -O2 %S/Inputs/jmp_opt{,2,3}.cpp -o %t
+RUN: %clangxx %cxxflags -O2 %S/Inputs/jmp_opt{,2,3}.cpp -o %t
 RUN: llvm-bolt -inline-small-functions %t -o %t.bolt
 RUN: llvm-objdump -d %t.bolt --print-imm-hex | FileCheck %s
 

bolt/test/X86/match-functions-with-call-graph.test

@@ -2,7 +2,7 @@
 
 # REQUIRES: system-linux
 # RUN: split-file %s %t
-# RUN: %clang %cflags %t/main.cpp -o %t.exe -Wl,-q -nostdlib
+# RUN: %clangxx %cxxflags %t/main.cpp -o %t.exe -Wl,-q -nostdlib
 # RUN: llvm-bolt %t.exe -o %t.out --data %t/yaml --profile-ignore-hash -v=1 \
 # RUN:   --dyno-stats --print-cfg --infer-stale-profile=1 --match-with-call-graph 2>&1 | FileCheck %s
 

bolt/test/pie.test

@@ -4,7 +4,7 @@
 ## on Linux systems where the host triple matches the target.
 REQUIRES: system-linux
 
-RUN: %clang %cflags -fPIC -pie %p/Inputs/jump_table_icp.cpp -o %t
+RUN: %clangxx %cxxflags -fPIC -pie %p/Inputs/jump_table_icp.cpp -o %t
 RUN: llvm-bolt %t -o %t.null 2>&1 | FileCheck %s
 
 CHECK: BOLT-INFO: shared object or position-independent executable detected

bolt/test/runtime/bolt-reserved.cpp

@@ -5,7 +5,7 @@
  * new sections.
  */
 
-// RUN: %clang %s -o %t.exe -Wl,-q
+// RUN: %clangxx %s -o %t.exe -Wl,-q
 // RUN: llvm-bolt %t.exe -o %t.bolt.exe 2>&1 | FileCheck %s
 // RUN: %t.bolt.exe
 
@@ -16,7 +16,7 @@
  * not enough for allocating new sections.
  */
 
-// RUN: %clang %s -o %t.tiny.exe -Wl,--no-eh-frame-hdr -Wl,-q -DTINY
+// RUN: %clangxx %s -o %t.tiny.exe -Wl,--no-eh-frame-hdr -Wl,-q -DTINY
 // RUN: not llvm-bolt %t.tiny.exe -o %t.tiny.bolt.exe 2>&1 | \
 // RUN:   FileCheck %s --check-prefix=CHECK-TINY