[docs] Corrected inaccuracies in Common Problems section.

Changed the language in LLVM_USE_LINKER to more strongly recommend LLD
and to specify that the GNU gold linker is only useful if LLD is
unavailable in binary form and it is the first build of LLVM. Added that
LLD will help when used on ELF-based platforms.

Corrected information in CMAKE_BUILD_TYPE regarding the Release build
type and enabling assertions.

Added option LLVM_ENABLE_ASSERTIONS and mentioned enabling this option
with a Release build as an alternative to using a Debug build.

Specified that the LLVM_OPTIMIZED_TABLEGEN
option is only for Debug builds, that the LLVM_USE_SPLIT_DWARF option
is only available on ELF host platforms, and that setting
CLANG_ENABLE_STATIC_ANALYZER to OFF only slightly improves build time.

These changes address comments made in D75425.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D77346