Refactor LongVectors.cpp to improve binary sizes (microsoft#7776)

This moves some things around so that less code is generated at compile
time and some information about data types and operations are looked up
at runtime.

For a debug build this reduces the size of the DLL by about 1.9MB, and
the PDB by about 8.1MB.

It also allows us to remove the `/bigobj` compiler flag.

Author: damyanp

tools/clang/unittests/HLSLExec/CMakeLists.txt

@@ -3,8 +3,6 @@
 find_package(TAEF REQUIRED)
 find_package(D3D12 REQUIRED) # Used for ExecutionTest.cpp.
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /bigobj")
-
 add_clang_library(ExecHLSLTests SHARED
   ExecutionTest.cpp
   ShaderOpTest.cpp