Add support for Xclang '-mrelax-all' and '-mconstructor-aliases' flags.

[Snektron]

This is basically the same implementation as #1547 and #1721.

Closes #2370.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.62%. Comparing base (9fb3841) to head (04e3f24).

Files with missing lines	Patch %	Lines
src/compiler/clang.rs	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2371      +/-   ##
==========================================
+ Coverage   67.60%   67.62%   +0.02%     
==========================================
  Files          65       65              
  Lines       35471    35488      +17     
==========================================
+ Hits        23979    23998      +19     
+ Misses      11492    11490       -2     

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