Tweak Clang 5/6 gcc toolchain versions

[RubenRBS]

Not meant to be merged yet, we need to make sure this time we get it right.
Any tests welcome

It fixes #903 if merged

[codecov]

Codecov Report

Merging #905 into master will decrease coverage by 5.2%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #905      +/-   ##
==========================================
- Coverage   62.04%   56.83%   -5.21%     
==========================================
  Files          64       42      -22     
  Lines        2914     2187     -727     
  Branches      548      421     -127     
==========================================
- Hits         1808     1243     -565     
+ Misses       1106      944     -162

Impacted Files	Coverage Δ
lib/compilers/golang.js	0% <0%> (-74.36%)	⬇️
lib/compiler-finder.js	0% <0%> (-42.03%)	⬇️
lib/base-compiler.js	5.14% <0%> (-5.26%)	⬇️
lib/compilers/pascal.js	50% <0%> (-1.58%)	⬇️
lib/languages.js	88.88% <0%> (-1.12%)	⬇️
lib/properties.js	98.24% <0%> (-0.48%)	⬇️
lib/aws.js	94.87% <0%> (-0.26%)	⬇️
lib/handlers/api.js	100% <0%> (ø)	⬆️
lib/asm-raw.js	0% <0%> (ø)	⬆️
lib/symbol-store.js	100% <0%> (ø)	⬆️
... and 37 more

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[partouf]

The problem I have with this kind of solution -- I mean you should probably merge this if it works and solves their issues, but -- is that it's not a proper solution.

I think 1. There should be some kind of proof (I can't find it on google) that this particular version of ClangC++ is tested and should work with libstdc++ version X.

And 2. It should probably be turned into a user option so people can use other compatible implementations and versions of libc++ and/or libc.

[mattgodbolt]

While I agree completely with @partouf 's comments, this might be an improvement already. It does have the potential to break existing URLs, admittedly. My gut instinct is to go with this though, and use the latest gcc major revision available at each clang publish. According to Wikipedia:

• clang 5 released on 7 Sep 2017
• clang 6 released on 8 March 2018

According to GCC's release page, GCC 7.3 was the latest build at both these releases. So I'm cool to merge this.