[lvm-windres] Try to match GNU windres regarding handling of unescaped quotes

Some background context: GNU windres invokes the preprocessor in
a subprocess. Some windres options are passed through to the
preproocessor, e.g. -D options for predefining defines.
When GNU windres passes these options onwards, it takes the options
in exact the form they are received (in argv or similar) and
assembles them into a single preprocessor command string which gets
interpreted by a shell (IIRC via the popen() function, or similar).

When LLVM invokes subprocesses, it does so via APIs that take
properly split argument vectors, to avoid needing to worry about
shell quoting/escaping/unescaping. But in the case of LLVM windres,
we have to emulate the effect of the shell parsing done by popen().

Most of the relevant cases are already taken care of here, but this
patch fixes an uncommon case encountered in
#57334.
(This case is uncommon since it doesn't do what one would want to;
the quotes need to be escaped more to work as intended through
the popen() shell).

Differential Revision: https://reviews.llvm.org/D146848

Author: mstorsjo

Diff for: llvm/test/tools/llvm-rc/windres-preproc.test

@@ -1,4 +1,4 @@
-; RUN: llvm-windres -### --include-dir %p/incdir1 --include %p/incdir2 "-DFOO1=\\\"foo bar\\\"" -UFOO2 -D FOO3 --preprocessor-arg "-DFOO4=\\\"baz baz\\\"" %p/Inputs/empty.rc %t.res | FileCheck %s --check-prefix=CHECK1
-; CHECK1: {{^}} "clang" "--driver-mode=gcc" "-target" "{{.*}}-{{.*}}{{mingw32|windows-gnu}}" "-E" "-xc" "-DRC_INVOKED" "{{.*}}empty.rc" "-o" "{{.*}}preproc-{{.*}}.rc" "-I" "{{.*}}incdir1" "-I" "{{.*}}incdir2" "-D" "FOO1=\"foo bar\"" "-U" "FOO2" "-D" "FOO3" "-DFOO4=\"baz baz\""{{$}}
+; RUN: llvm-windres -### --include-dir %p/incdir1 --include %p/incdir2 "-DFOO1=\\\"foo bar\\\"" -UFOO2 -D FOO3 --preprocessor-arg "-DFOO4=\\\"baz baz\\\"" -DFOO5=\"bar\" %p/Inputs/empty.rc %t.res | FileCheck %s --check-prefix=CHECK1
+; CHECK1: {{^}} "clang" "--driver-mode=gcc" "-target" "{{.*}}-{{.*}}{{mingw32|windows-gnu}}" "-E" "-xc" "-DRC_INVOKED" "{{.*}}empty.rc" "-o" "{{.*}}preproc-{{.*}}.rc" "-I" "{{.*}}incdir1" "-I" "{{.*}}incdir2" "-D" "FOO1=\"foo bar\"" "-U" "FOO2" "-D" "FOO3" "-DFOO4=\"baz baz\"" "-D" "FOO5=bar"{{$}}
 ; RUN: llvm-windres -### --preprocessor "i686-w64-mingw32-gcc -E -DFOO=\\\"foo\\ bar\\\"" %p/Inputs/empty.rc %t.res | FileCheck %s --check-prefix=CHECK2
 ; CHECK2: {{^}} "i686-w64-mingw32-gcc" "-E" "-DFOO=\"foo bar\"" "{{.*}}empty.rc" "-o" "{{.*}}preproc-{{.*}}.rc"{{$}}

Diff for: llvm/tools/llvm-rc/llvm-rc.cpp

@@ -322,6 +322,9 @@ std::string unescape(StringRef S) {
       else
         fatalError("Unterminated escape");
       continue;
+    } else if (S[I] == '"') {
+      // This eats an individual unescaped quote, like a shell would do.
+      continue;
     }
     Out.push_back(S[I]);
   }