[Multilib] Add -fmultilib-flag command-line option (#110658)

This patch is the second step to extend the current multilib system to
support the selection of library variants which do not correspond to
existing command-line options.

Proposal can be found in
https://discourse.llvm.org/t/rfc-multilib-custom-flags/81058

The multilib mechanism supports libraries that target code generation or
language options such as --target, -mcpu, -mfpu, -mbranch-protection.
However, some library variants are particular to features that do not
correspond to any command-line options. Examples include variants for
multithreading and semihosting.

This work introduces a way to instruct the multilib system to consider
these features in library selection.

The driver must be informed about the multilib custom flags with a new
command-line option.
```
-fmultilib-flag=C
```
Where the grammar for C is:
```
C -> option
option -> multithreaded | no-multithreaded | io-none | io-semihosting | io-linux-syscalls | ...
```
There must be one option instance for each flag specified:
```
-fmultilib-flag=multithreaded -fmultilib-flag=io-semihosting
```
Contradictory options are untied by *last one wins*.

These options are to be used exclusively by the multilib mechanism in
the Clang driver. Hence they are not forwarded to the compiler frontend.

Author: vhscampos

clang/include/clang/Driver/Options.td

@@ -5756,6 +5756,8 @@ def print_multi_directory : Flag<["-", "--"], "print-multi-directory">;
 def print_multi_lib : Flag<["-", "--"], "print-multi-lib">;
 def print_multi_flags : Flag<["-", "--"], "print-multi-flags-experimental">,
   HelpText<"Print the flags used for selecting multilibs (experimental)">;
+def fmultilib_flag : Joined<["-", "--"], "fmultilib-flag=">,
+  Visibility<[ClangOption]>;
 def print_multi_os_directory : Flag<["-", "--"], "print-multi-os-directory">,
   Flags<[Unsupported]>;
 def print_target_triple : Flag<["-", "--"], "print-target-triple">,

clang/lib/Driver/ToolChain.cpp

@@ -196,6 +196,15 @@ bool ToolChain::defaultToIEEELongDouble() const {
   return PPC_LINUX_DEFAULT_IEEELONGDOUBLE && getTriple().isOSLinux();
 }
 
+static void processMultilibCustomFlags(Multilib::flags_list &List,
+                                       const llvm::opt::ArgList &Args) {
+  for (const Arg *MultilibFlagArg :
+       Args.filtered(options::OPT_fmultilib_flag)) {
+    List.push_back(MultilibFlagArg->getAsString(Args));
+    MultilibFlagArg->claim();
+  }
+}
+
 static void getAArch64MultilibFlags(const Driver &D,
                                           const llvm::Triple &Triple,
                                           const llvm::opt::ArgList &Args,
@@ -246,6 +255,8 @@ static void getAArch64MultilibFlags(const Driver &D,
   if (ABIArg) {
     Result.push_back(ABIArg->getAsString(Args));
   }
+
+  processMultilibCustomFlags(Result, Args);
 }
 
 static void getARMMultilibFlags(const Driver &D,
@@ -313,6 +324,7 @@ static void getARMMultilibFlags(const Driver &D,
     if (Endian->getOption().matches(options::OPT_mbig_endian))
       Result.push_back(Endian->getAsString(Args));
   }
+  processMultilibCustomFlags(Result, Args);
 }
 
 static void getRISCVMultilibFlags(const Driver &D, const llvm::Triple &Triple,

clang/test/Driver/print-multi-selection-flags.c

@@ -90,3 +90,10 @@
 // CHECK-RV32E-ORDER: --target=riscv32-unknown-none-elf
 // CHECK-RV32E-ORDER: -mabi=ilp32e
 // CHECK-RV32E-ORDER: -march=rv32e{{[0-9]+p[0-9]+}}_c{{[0-9]+p[0-9]+}}_zicsr{{[0-9]+p[0-9]+}}
+
+// RUN: %clang -print-multi-flags-experimental --target=armv8m.main-none-eabi -fmultilib-flag=foo -fmultilib-flag=bar | FileCheck --check-prefixes=CHECK-MULTILIB-CUSTOM-FLAG,CHECK-ARM-MULTILIB-CUSTOM-FLAG %s
+// RUN: %clang -print-multi-flags-experimental --target=aarch64-none-eabi     -fmultilib-flag=foo -fmultilib-flag=bar | FileCheck --check-prefixes=CHECK-MULTILIB-CUSTOM-FLAG,CHECK-AARCH64-MULTILIB-CUSTOM-FLAG %s
+// CHECK-ARM-MULTILIB-CUSTOM-FLAG:     --target=thumbv8m.main-unknown-none-eabi
+// CHECK-AARCH64-MULTILIB-CUSTOM-FLAG: --target=aarch64-unknown-none-eabi
+// CHECK-MULTILIB-CUSTOM-FLAG-DAG:     -fmultilib-flag=foo
+// CHECK-MULTILIB-CUSTOM-FLAG-DAG:     -fmultilib-flag=bar