Bump to mono/llvm@349752c4 (#2453)

Fixes: #2290

Executing `llc.exe` on Windows would result in a crash:

	> msbuild TodoAzure.Droid.csproj /t:SignAndroidPackage /p:Configuration=Release /p:AotAssemblies=true /p:EnableLlvm=true
	...
	[aot-compiler stdout] Mono Ahead of Time compiler - compiling assembly C:\agent\_work\r8\a\xamarin-forms-samples\WebServices\TodoAzurePush\Droid\obj\Release\81\android\assets\PCLCrypto.dll (TaskId:288)
	[aot-compiler stdout] AOTID 5E31DBA6-D7C4-378F-663E-D6E8726A1DF9 (TaskId:288)
	[aot-compiler stdout] Executing opt: "C:\PROGRA~2\MICROS~1\2017\ENTERP~1\MSBuild\Xamarin\Android\opt" -f -O2 -disable-tail-calls -o "C:\agent\_work\r8\a\XAMARI~2\WEBSER~1\TO3284~1\Droid\obj\Release\81\aot\ARMEAB~1\PCLCrypto.dll\temp.opt.bc" "C:\agent\_work\r8\a\XAMARI~2\WEBSER~1\TO3284~1\Droid\obj\Release\81\aot\ARMEAB~1\PCLCrypto.dll\temp.bc" (TaskId:288)
	[AOT] MONO_PATH="C:\agent\_work\r8\a\xamarin-forms-samples\WebServices\TodoAzurePush\Droid\obj\Release\81\android\assets" MONO_ENV_OPTIONS="" "C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\Xamarin\Android\cross-arm" --llvm --aot=outfile=C:\agent\_work\r8\a\XAMARI~2\WEBSER~1\TO3284~1\Droid\obj\Release\81\aot\ARMEAB~1\libaot-Xamarin.Android.Support.Animated.Vector.Drawable.dll.so,asmwriter,mtriple=armv7-linux-gnueabi,tool-prefix=C:\PROGRA~2\Android\ANDROI~2\NDK-BU~1\TOOLCH~1\ARM-LI~1.9\prebuilt\WINDOW~1\bin\arm-linux-androideabi-,ld-flags=C:\PROGRA~2\Android\ANDROI~2\NDK-BU~1\TOOLCH~1\ARM-LI~1.9\prebuilt\WINDOW~1\lib\gcc\ARM-LI~1\49696B~1.X\libgcc.a;C:\PROGRA~2\Android\ANDROI~2\NDK-BU~1\PLATFO~1\ANDROI~2\arch-arm\usr\lib\libc.so;C:\PROGRA~2\Android\ANDROI~2\NDK-BU~1\PLATFO~1\ANDROI~2\arch-arm\usr\lib\libm.so,llvm-path=C:\PROGRA~2\MICROS~1\2017\ENTERP~1\MSBuild\Xamarin\Android,temp-path=C:\agent\_work\r8\a\XAMARI~2\WEBSER~1\TO3284~1\Droid\obj\Release\81\aot\ARMEAB~1\Xamarin.Android.Support.Animated.Vector.Drawable.dll C:\agent\_work\r8\a\xamarin-forms-samples\WebServices\TodoAzurePush\Droid\obj\Release\81\android\assets\Xamarin.Android.Support.Animated.Vector.Drawable.dll (TaskId:288)
	[aot-compiler stdout] Executing llc: "C:\PROGRA~2\MICROS~1\2017\ENTERP~1\MSBuild\Xamarin\Android\llc"  -mattr=+vfp2,-neon,+d16 -asm-verbose=false -mtriple=armv7-linux-gnueabi -disable-gnu-eh-frame -enable-mono-eh-frame -mono-eh-frame-symbol=mono_aot_PCLCrypto_eh_frame -disable-tail-calls -relocation-model=pic -o "C:\agent\_work\r8\a\XAMARI~2\WEBSER~1\TO3284~1\Droid\obj\Release\81\aot\ARMEAB~1\PCLCrypto.dll\temp-llvm.s" "C:\agent\_work\r8\a\XAMARI~2\WEBSER~1\TO3284~1\Droid\obj\Release\81\aot\ARMEAB~1\PCLCrypto.dll\temp.opt.bc" (TaskId:288)
	[aot-compiler stderr] Wrote crash dump file "C:\Users\XAMARI~1\AppData\Local\Temp\llc.exe-b94cbe.dmp" (TaskId:288)
	[aot-compiler stderr] AOT of image C:\agent\_work\r8\a\xamarin-forms-samples\WebServices\TodoAzurePush\Droid\obj\Release\81\android\assets\PCLCrypto.dll failed. (TaskId:288)

The cause of the crash was due to a [mingw i686 optimization bug][0].

Bump to mono/llvm@349752c4 which *doesn't* optimize
`findCommon()`, thus fixing the crash.

[0]: mono/llvm#19

Author: lateralusX