fix "code will never be executed" under clang + ccache

[lesurp]

Description

The issue, for some otherworldly reason, only seems to appear when using ccache, with the -c flag (see relevant issue).

When compiling any test such as:

#define CATCH_CONFIG_MAIN
#include "catch.hpp"

TEST_CASE("foo", "[bar]")
{
    double a = 2.0;
    double b = 10.0;
    CHECK(a * 5.0/3.0  == b / 3.0);
}

Compiling it (with ccache):

ccache /usr/bin/clang++     -Wunreachable-code   -std=gnu++11 -c /absolute/path/test.cpp
/absolute/path/test.cpp:8:161: warning: code will never be executed [-Wunreachable-code]
 } catch(...) { catchAssertionHandler.handleUnexpectedInflightException(); } catchAssertionHandler.complete(); } while( (void)0, false && static_cast<bool>( !!(a * 5.0/3.0 == b / 3.0) ) );
                                                                                                                                                                ^
/absolute/path/test.cpp:8:130: note: silence by adding parentheses to mark code as explicitly dead
 } catch(...) { catchAssertionHandler.handleUnexpectedInflightException(); } catchAssertionHandler.complete(); } while( (void)0, false && static_cast<bool>( !!(a * 5.0/3.0 == b / 3.0) ) );

Why it is worth fixing:

1. It creates very noisy warnings, and cannot really be avoided because...
2. CMake always compiles objects separately by default (therefore with the -c flag)
3. ... and even if it didn't, since we should compile Catch2's main separately, we'd still need to compile the objects separately

[codecov]

Codecov Report

Merging #1540 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1540   +/-   ##
======================================
  Coverage    80.6%   80.6%           
======================================
  Files         121     121           
  Lines        3386    3386           
======================================
  Hits         2729    2729           
  Misses        657     657