[BOLT] Fix handling of code references from unmodified code

In lite mode (default for X86), BOLT optimizes and relocates functions
with profile. The rest of the code is preserved, but if it references
relocated code such references have to be updated. The update is handled
by scanExternalRefs() function. Note that we cannot solely rely on
relocations written by the linker, as not all code references are
exposed to the linker. Additionally, the linker can modify certain
instructions and relocations will no longer match the code.

With this change, start using symbolic disassembler for scanning code
for references in scanExternalRefs(). Unlike the previous approach, the
symbolizer properly detects and creates references for instructions with
multiple/ambiguous symbolic operands and handles cases where a
relocation doesn't match any operand. See test cases for examples.

Reviewed By: Amir

Differential Revision: https://reviews.llvm.org/D152631