No longer disable wd4146, fix warnings, and disable this warning when
including LLVM headers in LLVMReactor.cpp, as it trips for specific
headers like MathExtras.h, StringExtras.h.

Fixes ANGLE roll.

Bug: angleproject:4034
Change-Id: I102f4c18ac47c2db8abddf1b4ceace7987e6df57
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/37488
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>