`<random>`: Consider using Boost.Math's `lgamma()` implementation

[StephanTLavavej]

<random>'s poisson_distribution and binomial_distribution need a "log gamma" function. In #5891, I made them use the UCRT's C99 lgamma() instead of the STL's _XLgamma(). (While _XLgamma() had to be retained for bincompat, I replaced its terrible implementation with UCRT lgamma() too.)

Boost.Math has an even more accurate implementation of lgamma(). In theory, we could switch the STL's _XLgamma() over to wrapping Boost.Math, and then begin calling it again.

As _XLgamma() is compiled into the STL's original DLL and static LIB, I believe this is blocked by #362. We need the ability to change Boost.Math's standalone namespace to avoid conflicting with user Boost usage. This problem is annoying for Special Math but would be more severe if we introduced it into the STL's original static LIB.

[YexuanXiao]

How about using llvm-objcopy? llvm-objcopy makes it easy to modify symbols in libs. For example:

$RawNS = "@boost"
$NewNS = "@_Boost"

$LibPath = "special_math.cpp.obj"
$LibPathNew = "special_math.cpp.1.obj"

$tempRulesFile = "temp_rules.txt"

$symbols = llvm-nm -j $LibPath

$rules = @()

foreach ($symbol in $symbols) {
    if ($symbol.Contains($RawNS)) {
        $newSymbol = $symbol.Replace($RawNS, $NewNS)
        $rules += "$symbol $newSymbol"
    }
}

if ($rules.Count -gt 0) {
    Set-Content -Path $tempRulesFile -Value $rules
    llvm-objcopy --redefine-syms $tempRulesFile $LibPath $LibPathNew
    Remove-Item $tempRulesFile
}

At least until boost.math supports changing the namespace, this method can easily fix the issue.

[StephanTLavavej]

We're not really set up to take a dependency on LLVM tools in the MSVC-internal build.