[clang] Add a -canonical-prefixes option

In https://reviews.llvm.org/D47480 I complained that there's no positive
form of this flag, so let's add one :)

https://gcc.gnu.org/PR29931 also has a pending patch to add the positive
form to gcc (but there's admittedly not a lot of movement on that bug).

This doesn't change any defaults.

Differential Revision: https://reviews.llvm.org/D108818

Author: nico

clang/include/clang/Driver/Options.td

@@ -3677,8 +3677,10 @@ def multi__module : Flag<["-"], "multi_module">;
 def multiply__defined__unused : Separate<["-"], "multiply_defined_unused">;
 def multiply__defined : Separate<["-"], "multiply_defined">;
 def mwarn_nonportable_cfstrings : Flag<["-"], "mwarn-nonportable-cfstrings">, Group<m_Group>;
+def canonical_prefixes : Flag<["-"], "canonical-prefixes">, Flags<[HelpHidden, CoreOption]>,
+  HelpText<"Use absolute paths for invoking subcommands (default)">;
 def no_canonical_prefixes : Flag<["-"], "no-canonical-prefixes">, Flags<[HelpHidden, CoreOption]>,
-  HelpText<"Use relative instead of canonical paths">;
+  HelpText<"Use relative paths for invoking subcommands">;
 def no_cpp_precomp : Flag<["-"], "no-cpp-precomp">, Group<clang_ignored_f_Group>;
 def no_integrated_cpp : Flag<["-", "--"], "no-integrated-cpp">, Flags<[NoXarchOption]>;
 def no_pedantic : Flag<["-", "--"], "no-pedantic">, Group<pedantic_Group>;

clang/lib/Driver/Driver.cpp

@@ -1091,7 +1091,8 @@ Compilation *Driver::BuildCompilation(ArrayRef<const char *> ArgList) {
   // Silence driver warnings if requested
   Diags.setIgnoreAllWarnings(Args.hasArg(options::OPT_w));
 
-  // -no-canonical-prefixes is used very early in main.
+  // -canonical-prefixes, -no-canonical-prefixes are used very early in main.
+  Args.ClaimAllArgs(options::OPT_canonical_prefixes);
   Args.ClaimAllArgs(options::OPT_no_canonical_prefixes);
 
   // f(no-)integated-cc1 is also used very early in main.

clang/test/Driver/no-canonical-prefixes.c

@@ -10,8 +10,20 @@
 // RUN: rm -f %t.fake
 // RUN: ln -sf %t.real %t.fake
 // RUN: cd %t.fake
-// RUN: ./test-clang -v -S %s 2>&1 | FileCheck --check-prefix=CANONICAL %s
-// RUN: ./test-clang -v -S %s -no-canonical-prefixes 2>&1 | FileCheck --check-prefix=NON-CANONICAL %s
+// RUN: ./test-clang -v -S %s 2>&1 \
+// RUN:     | FileCheck --check-prefix=CANONICAL %s
+// RUN: ./test-clang -v -S %s 2>&1 \
+// RUN:     -no-canonical-prefixes \
+// RUN:     | FileCheck --check-prefix=NON-CANONICAL %s
+// RUN: ./test-clang -v -S %s 2>&1 \
+// RUN:     -no-canonical-prefixes \
+// RUN:     -canonical-prefixes \
+// RUN:     | FileCheck --check-prefix=CANONICAL %s
+// RUN: ./test-clang -v -S %s 2>&1 \
+// RUN:     -no-canonical-prefixes \
+// RUN:     -canonical-prefixes \
+// RUN:     -no-canonical-prefixes \
+// RUN:     | FileCheck --check-prefix=NON-CANONICAL %s
 //
 // FIXME: This should really be '.real'.
 // CANONICAL: InstalledDir: {{.*}}.fake

clang/tools/driver/driver.cpp

@@ -416,10 +416,10 @@ int main(int Argc, const char **Argv) {
     // Skip end-of-line response file markers
     if (Args[i] == nullptr)
       continue;
-    if (StringRef(Args[i]) == "-no-canonical-prefixes") {
+    if (StringRef(Args[i]) == "-canonical-prefixes")
+      CanonicalPrefixes = true;
+    else if (StringRef(Args[i]) == "-no-canonical-prefixes")
       CanonicalPrefixes = false;
-      break;
-    }
   }
 
   // Handle CL and _CL_ which permits additional command line options to be