Expose tool pass-through mechanisms

[woodruffw]

The CompilerTool class should detect the tool pass-through mechanisms used by both Clang and GCC and expose them as appropriate.

Some examples:

• Assembler passthrough: -Wa -flag1,-flag2,-flag3 should be exposed as assembler_args
• Linker passthrough: -Wl -flag1,-flag2,-flag2 should be exposed as linker_args
• Preprocessor passthroughL: -Wp -flag1,-flag2,-flag2 should be exposed as preprocessor_args

Notes:

• These pass-through mechanisms also support a no-space form like -Wa,-flag1, so this also needs to be handled.
• -Xassembler, -Xlinker, and -Xpreprocessor each behave the same as their -W counterparts above, although they don't appear to support the no-space form.
• Clang also supports -Xanalyzer, for the Clang-specific analyzer and -Xclang for passing options to cc1.
• Clang also supports -mllvm for forwarding LLVM-specific arguments. This form appears to only take one argument at a time.
• Clang also supports --for-linker <arg> and --for-linker=<arg> as equivalents to -Xlinker <arg>. -X <arg> might also be equivalent.

[woodruffw]

We should also support linker groups for CompileTools, since apparently they can embed source inputs and not just objects: SRI-CSL/gllvm#48