$ clang++ a.c 
clang-5.0: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
/usr/bin/ld: cannot find -lstdc++
clang-5.0: error: linker command failed with exit code 1 (use -v to see invocation)

$ spack find -dlv llvm@5.0.0
==> 1 installed packages.
-- linux-ubuntu16.04-x86_64 / gcc@6.4.0 -------------------------
vvs4bks    llvm@5.0.0+all_targets build_type=Release +clang+compiler-rt+gold+internal_unwind+libcxx~link_dylib+lldb+polly~shared_libs
atc354d        ^binutils@2.28+gold~libiberty patches=0a0ca84889b6727dc0cc88650f55fd79452dddba762674a83a153374792fdaac ~plugins
z4rue2x            ^gettext@0.19.8.1+bzip2+curses+git~libunistring+libxml2 patches=9acdb4e73f67c241b5ef32505c9ddf7cf6884ca8ea661692f21dca28483b04b8 +tar+xz
bmyxoqb                ^bzip2@1.0.6+shared
zqliw5y                ^libxml2@2.9.4~python
2tho6an                    ^xz@5.2.3
yvlwvyz                    ^zlib@1.2.11+pic+shared
jtmz6tz                ^ncurses@6.0 patches=4110a40613b800da2b2888c352b64c75a82809d48341061e4de5861e8b28423f,f84b2708a42777aadcc7f502a261afe10ca5646a51c1ef8b5e60d2070d926b57 ~symlinks
f3sg6dt                ^tar@1.29 patches=08921fcbd732050c74ddf1de7d8ad95ffdbc09f8b4342456fa2f6a0dd02a957c,5c314db58d005043bf407abaf25eb9823b9032a22fd12a0b142d4bf548130fa4,d428578be7fb99b831eb61e53b8d88a859afe08b479a21238180899707d79ce4,e34d2470461037933737c0a60fbcc5d5ea905fefb6b414b4a2e88a440fcae23a
hrqaevt        ^libedit@3.1-20170329
nreklkn        ^py-lit@0.5.0
n6pbabv            ^python@2.7.14 patches=123082ab3483ded78e86d7c809e98a804b3465b4683c96bd79a2fd799f572244 +pic+shared~tk~ucs4
ztgs5rp                ^openssl@1.0.2k
323dute                ^readline@7.0
25axhq6                ^sqlite@3.20.0
ljvueky        ^swig@3.0.12
nx2htpr            ^pcre@8.40~jit+utf