[clang][tests] Fix a typo in wasm-toolchain.cpp tests

[tbaeder]

Summary

This seemed odd to me while looking at the tests.

Note that I couldn't make the tests fail when providing (other) invalid values for the --stdlib parameter here anyway, but c++ is not used anywhere else as far as I can see.

clang/test/Driver/wasm-toolchain.cpp

 // A basic clang -cc1 command-line. WebAssembly is somewhat special in
 // enabling -fvisibility=hidden by default.
 
 // RUN: %clangxx %s -### -no-canonical-prefixes -target wasm32-unknown-unknown 2>&1 \
 // RUN:   | FileCheck -check-prefix=CC1 %s
 // CC1: clang{{.*}} "-cc1" "-triple" "wasm32-unknown-unknown" {{.*}} "-fvisibility" "hidden" {{.*}}
 
 // Ditto, but ensure that a user -fvisibility=default disables the default
 // -fvisibility=hidden.
 
 // RUN: %clangxx %s -### -target wasm32-unknown-unknown -fvisibility=default 2>&1 \
 // RUN:   | FileCheck -check-prefix=FVISIBILITY_DEFAULT %s
 // FVISIBILITY_DEFAULT-NOT: hidden
 
 // A basic C++ link command-line with unknown OS.
 
-// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo --stdlib=c++ %s 2>&1 \
+// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo --stdlib=libc++ %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=LINK %s
 // LINK: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK: wasm-ld{{.*}}" "-L/foo/lib" "crt1.o" "[[temp]]" "-lc++" "-lc++abi" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
 
 // A basic C++ link command-line with optimization with unknown OS.
 
-// RUN: %clangxx -### -O2 -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s --stdlib=c++ 2>&1 \
+// RUN: %clangxx -### -O2 -no-canonical-prefixes -target wasm32-unknown-unknown --sysroot=/foo %s --stdlib=libc++ 2>&1 \
 // RUN:   | FileCheck -check-prefix=LINK_OPT %s
 // LINK_OPT: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK_OPT: wasm-ld{{.*}}" "-L/foo/lib" "crt1.o" "[[temp]]" "-lc++" "-lc++abi" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
 
 // A basic C++ link command-line with known OS.
 
-// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo --stdlib=c++ %s 2>&1 \
+// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo --stdlib=libc++ %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=LINK_KNOWN %s
 // LINK_KNOWN: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK_KNOWN: wasm-ld{{.*}}" "-L/foo/lib/wasm32-wasi" "crt1.o" "[[temp]]" "-lc++" "-lc++abi" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
 
 // A basic C++ link command-line with optimization with known OS.
 
-// RUN: %clangxx -### -O2 -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo %s --stdlib=c++ 2>&1 \
+// RUN: %clangxx -### -O2 -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo %s --stdlib=libc++ 2>&1 \
 // RUN:   | FileCheck -check-prefix=LINK_OPT_KNOWN %s
 // LINK_OPT_KNOWN: clang{{.*}}" "-cc1" {{.*}} "-o" "[[temp:[^"]*]]"
 // LINK_OPT_KNOWN: wasm-ld{{.*}}" "-L/foo/lib/wasm32-wasi" "crt1.o" "[[temp]]" "-lc++" "-lc++abi" "-lc" "{{.*[/\\]}}libclang_rt.builtins-wasm32.a" "-o" "a.out"
 
 // A basic C++ compile command-line with known OS.
 
-// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo --stdlib=c++ %s 2>&1 \
+// RUN: %clangxx -### -no-canonical-prefixes -target wasm32-wasi --sysroot=/foo --stdlib=libc++ %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=COMPILE %s
 // COMPILE: clang{{.*}}" "-cc1"
 // COMPILE: "-resource-dir" "[[RESOURCE_DIR:[^"]*]]"
 // COMPILE: "-isysroot" "/foo"
 // COMPILE: "-internal-isystem" "/foo/include/wasm32-wasi/c++/v1"
 // COMPILE: "-internal-isystem" "/foo/include/c++/v1"
 // COMPILE: "-internal-isystem" "[[RESOURCE_DIR]]{{(/|\\\\)}}include"
 // COMPILE: "-internal-isystem" "/foo/include/wasm32-wasi"
 // COMPILE: "-internal-isystem" "/foo/include"