[msan] Add MSan instrumentation support for Hexagon (#189122)
Add MemorySanitizer instrumentation pass support for Hexagon Linux. This
is the codegen/instrumentation side; the compiler-rt runtime changes are
in a separate patch.
The shadow memory layout uses XOR-based mapping with XorMask=0x20000000
and OriginBase=0x50000000, designed to fit within the 32-bit address
space.
VarArg handling uses VarArgGenericHelper with VAListTagSize=12, matching
the Hexagon ABI where va_list is a three-pointer struct {
current_reg_area, reg_area_end, overflow_area }.
