Merge #19094: build: Only allow ASCII identifiers

399d84d build: Only allow ASCII identifiers (Wladimir J. van der Laan)

Pull request description:

  While emoji and other symbols in C++ identifers (as accepted by newer compilers) are fun, they might create confusion during code review, for example because some symbols look very similar. Forbid such extended identifiers for now.

  This is done by providing `-fno-extended-identifiers`. Thanks to sipa for suggesting this compiler flag.

ACKs for top commit:
  practicalswift:
    ACK 399d84d -- patch looks correct
  promag:
    ACK 399d84d.
  jonatack:
    ACK 399d84d
  fanquake:
    ACK 399d84d - seems like a good sanity check to enable.

Tree-SHA512: 62bfbe8c7e0284ed505c2c8789c1ae74997202d90595f298c2ee1917e5d69fa9b7196a9404ba2cff61f3162b2bbb5616a1591bed3f0534c58617e22009291933

Author: fanquake

configure.ac

@@ -422,6 +422,9 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wdeprecated-copy],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-deprecated-copy"],,[[$CXXFLAG_WERROR]])
 fi
 
+dnl Don't allow extended (non-ASCII) symbols in identifiers. This is easier for code review.
+AX_CHECK_COMPILE_FLAG([-fno-extended-identifiers],[[CXXFLAGS="$CXXFLAGS -fno-extended-identifiers"]],,[[$CXXFLAG_WERROR]])
+
 enable_sse42=no
 enable_sse41=no
 enable_avx2=no