Add "toolchain-identifier" attribute to cc_toolchain rule

Currently the selection of C++ toolchain configuration depends on 2 different things:
1. CROSSTOOL file: a proto text file that contains multiple CrosstoolConfig.CToolchain messages, out of which we want to select the appropriate CToolchain;
2. cc_toolchain_suite rule, specified by --crosstool_top option: this rule contains "toolchains" map attribute. The map's keys are of type <cpu>|<compiler>, or just <cpu>, and the values are labels of cc_toolchain rules (which contain additional C++ information).

If there is an entry in cc_toolchain_suite.toolchains that corresponds to the --cpu and --compiler options, we use it, otherwise we loop through all the CToolchains in the CROSSTOOL file, select the one that corresponds to the --cpu and --compiler options, and then get the cc_toolchain label from cc_toolchain_suite.toolchains[toolchain.targetCpu|toolchain.compiler].
In both cases we read the CROSSTOOL file and pass all its information forward, to be used in creation of CppConfiguration and CcToolchainProvider creation.

As part of the efforts of rewriting CROSSTOOL in Skylark, we need to make obtaining the cc_toolchain label independent of the CROSSTOOL file and toolchain selection. As a step towards that goal, we add a new "toolchain_identifier" attribute to cc_toolchain, which uniquely identifies a CToolchain in the CROSSTOOL file.

Now the process of getting the CToolchain goes as follows:
Check for existence of cc_toolchain_suite.toolchains[<cpu>|<compiler>], if --compiler is specified, otherwise check for cc_toolchain_suite.toolchains[<cpu>].

1. if a value is found, load the cc_toolchain rule and look for the toolchain_identifier attribute.
    1.a if the attribute exists, loop through all the CToolchains in CROSSTOOL and select the one with the matching toolchain identifier.
    1.b otherwise fall back to selecting the CToolchain from CROSSTOOL by matching the --cpu and --compiler values.

2. If a value is not found, select the CToolchain from CROSSTOOL by matching the --cpu and --compiler values, and construct the key as follows: <toolchain.cpu>|<toolchain.compiler>.

In the future we will get rid of 2. by making sure that cc_toolchain_suite.toolchains[<cpu>|<compiler>] and cc_toolchain_suite.toolchains[<cpu>] are always defined.
1.b will be removed by making the cc_toolchain.toolchain_identifier attribute mandatory
After this deriving the cc_toolchain label will be independent of the CROSSTOOL file

1.a will ultimately be replaced by an attribute that points to a skylark_crosstool rule, that will contain all the information that CROSSTOOL contains, allowing us to remove CROSSTOOL altogether.

Work towards issue #5380
RELNOTES: None.
PiperOrigin-RevId: 200388550

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